Method and apparatus for transmitting response frame based on type in a high efficiency wireless lan

ABSTRACT

The present disclosure relates to a method and apparatus for transmitting a response frame based on a type in a High Efficiency Wireless Local Area Network (WLAN) (HEW). According to an aspect, a method for transmitting an uplink frame by a station (STA) to an access point (AP) in a WLAN may be provided. The method may include receiving, from the AP, a downlink frame including information related to a type of the uplink frame, the type of the uplink frame including a single-user (SU) type and a multiple-user (MU) type; and transmitting, to the AP, the uplink frame having a type determined based on the information related to the type of the uplink frame, wherein, when the type of the uplink frame corresponds to the MU type, the uplink frame is simultaneously transmitted by a plurality of STAs including the STA and at least one other STA.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefits of U.S. Provisional Application No.62/074,514, filed on Nov. 3, 2014, and U.S. Provisional Application No.62/080,026, filed on Nov. 14, 2014, which are hereby incorporated byreference as if fully set forth herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a Wireless Local Area Network (WLAN),and more particularly, to a method, apparatus, and software fortransmitting a response frame according to a type in a High EfficiencyWLAN (HEW), and a recording medium that stores the software.

2. Discussion of the Related Art

Along with the recent development of information and telecommunicationtechnology, various wireless communication techniques have beendeveloped. Among them, the WLAN enables a user to wirelessly access theInternet based on radio frequency technology in a home, an office, or aspecific service area using a portable terminal such as a PersonalDigital Assistant (PDA), a laptop computer, a Portable Multimedia Player(PMP), a smartphone, etc.

To overcome limitations in communication speed that the WLAN faces, therecent technical standards have introduced a system that increases thespeed, reliability, and coverage of a wireless network. For example, theInstitute of Electrical and Electronics Engineers (IEEE) 802.11nstandard has introduced Multiple Input Multiple Output (MIMO) that isimplemented using multiple antennas at both a transmitter and a receiverin order to support High Throughput (HT) at a data processing rate of upto 540 Mbps, minimize transmission errors, and optimize data rates.

In recent times, to support increased numbers of devices supportingWLAN, such as smartphones, more Access Points (APs) have been deployed.Despite increase in use of WLAN devices supporting the Institute ofElectrical and Electronics Engineers (IEEE) 802.11 ac standard, thatprovide high performance relative to WLAN devices supporting the legacyIEEE 802.11g/n standard, a WLAN system supporting higher performance isrequired due to WLAN users' increased use of high volume content such asa ultra high definition video. Although a conventional WLAN system hasaimed at increase of bandwidth and improvement of a peak transmissionrate, actual users thereof could not feel drastic increase of suchperformance.

In a task group called IEEE 802.11 ax, High Efficiency WLAN (HEW)standardization is under discussion. The HEW aims at improvingperformance felt by users demanding high-capacity, high-rate serviceswhile supporting simultaneous access of numerous stations in anenvironment in which a plurality of APs is densely deployed and coverageareas of APs overlap.

However, there is no specified method for protecting a transmitted frameand no specified method for determining the type of a response frame ina HEW.

SUMMARY OF THE INVENTION

Objects of the present invention is to provide a method for protecting atransmitted frame and a method for determining the type of a responseframe in a High Efficiency WLAN (HEW).

The objects of the present invention are not limited to the foregoingdescriptions, and additional objects will become apparent to thosehaving ordinary skill in the pertinent art to the present inventionbased upon the following descriptions.

In an aspect of the present invention, a method for transmitting anuplink frame by a station (STA) to an access point (AP) in a WLAN may beprovided. The method may include receiving, from the AP, a downlinkframe including information related to a type of the uplink frame, thetype of the uplink frame including a single-user (SU) type and amultiple-user (MU) type; and transmitting, to the AP, the uplink framehaving a type determined based on the information related to the type ofthe uplink frame, wherein, when the type of the uplink frame correspondsto the MU type, the uplink frame is simultaneously transmitted by aplurality of STAs including the STA and at least one other STA.

In another aspect of the present invention, a method for receiving anuplink frame by an AP from at least one STA in a WLAN may be provided.The method may include transmitting, to the at least one STA, a downlinkframe including information related to a type of the uplink frame, thetype of the uplink frame including a single-user (SU) type and amultiple-user (MU) type; and receiving, from the at least one STA, theuplink frame having a type determined based on the information relatedto the type of the uplink frame, wherein, when the type of the uplinkframe corresponds to the MU type, the uplink frame is simultaneouslytransmitted by a plurality of STAs including the at least one STA.

In another aspect of the present invention, a STA apparatus fortransmitting an uplink frame to an AP in a WLAN may be provided. The STAapparatus may include a baseband processor, a Radio Frequency (RF)transceiver, a memory, etc. The baseband processor may be configured toreceive, from the AP, a downlink frame including information related toa type of the uplink frame, the type of the uplink frame including asingle-user (SU) type and a multiple-user (MU) type; and transmit, tothe AP, the uplink frame having a type determined based on theinformation related to the type of the uplink frame, wherein, when thetype of the uplink frame corresponds to the MU type, the uplink frame issimultaneously transmitted by a plurality of STAs including the STA andat least one other STA.

In another aspect of the present invention, an AP apparatus forreceiving an uplink frame from at least one STA in a WLAN may beprovided. The AP apparatus may include a baseband processor, an RFtransceiver, a memory, etc. The baseband processor may be configured totransmit, to the at least one STA, a downlink frame includinginformation related to a type of the uplink frame, the type of theuplink frame including a single-user (SU) type and a multiple-user (MU)type; and receive, from the at least one STA, the uplink frame having atype determined based on the information related to the type of theuplink frame, wherein, when the type of the uplink frame corresponds tothe MU type, the uplink frame is simultaneously transmitted by aplurality of STAs including the at least one STA.

In another aspect of the present invention, a software orcomputer-readable medium having instructions executable for an STA totransmit an uplink frame to an AP in a WLAN may be provided. Theexecutable instructions may cause the STA to receive, from the AP, adownlink frame including information related to a type of the uplinkframe, the type of the uplink frame including a single-user (SU) typeand a multiple-user (MU) type; and transmit, to the AP, the uplink framehaving a type determined based on the information related to the type ofthe uplink frame, wherein, when the type of the uplink frame correspondsto the MU type, the uplink frame is simultaneously transmitted by aplurality of STAs including the STA and at least one other STA.

In another aspect of the present invention, a software orcomputer-readable medium having instructions executable for an AP toreceive an uplink frame from at least one STA in a WLAN may be provided.The executable instructions may cause the AP to transmit, to the atleast one STA, a downlink frame including information related to a typeof the uplink frame, the type of the uplink frame including asingle-user (SU) type and a multiple-user (MU) type; and receive, fromthe at least one STA, the uplink frame having a type determined based onthe information related to the type of the uplink frame, wherein, whenthe type of the uplink frame corresponds to the MU type, the uplinkframe is simultaneously transmitted by a plurality of STAs including theat least one STA.

It is to be understood that both the foregoing summarized features areexemplary aspects of the following detailed description of the presentinvention without limiting the scope of the present invention.

According to the present invention, a method for protecting atransmitted frame and a method for determining the type of a responseframe in a HEW can be provided.

The advantages of the present invention are not limited to the foregoingdescriptions, and additional advantages will become apparent to thosehaving ordinary skill in the pertinent art to the present inventionbased upon the following descriptions.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this application, illustrate embodiment(s) of the invention andtogether with the description serve to explain the principle of theinvention. In the drawings:

FIG. 1 is a block diagram of a Wireless Local Area Network (WLAN)device;

FIG. 2 is a schematic block diagram of an exemplary transmitting signalprocessing unit in a WLAN;

FIG. 3 is a schematic block diagram of an exemplary receiving signalprocessing unit in a WLAN;

FIG. 4 depicts a relationship between InterFrame Spaces (IFSs);

FIG. 5 is a conceptual diagram illustrating a procedure for transmittinga frame in Carrier Sense Multiple Access with Collision Avoidance(CSMA/CA) for avoiding collisions between frames in a channel;

FIG. 6 depicts an exemplary frame structure in a WLAN system;

FIG. 7 depicts an exemplary HE PPDU frame format.

FIG. 8 depicts an exemplary High Efficiency (HE) Physical layer ProtocolData Unit (PPDU) frame format according to the present invention;

FIG. 9 depicts subchannel allocation in a HE PPDU frame format accordingto the present invention;

FIG. 10 depicts a subchannel allocation method according to the presentinvention;

FIG. 11 depicts the starting and ending points of an High EfficiencyLong Training Field (HE-LTF) field in a HE PPDU frame format accordingto the present invention;

FIG. 12 depicts a High Efficiency SIGnal B (HE-SIG-B) field and a HighEfficiency SIGnal C (HE-SIG-C) field in the HE PPDU frame formataccording to the present invention;

FIG. 13 depicts another example of a HE PPDU frame format according tothe present invention;

FIGS. 14 and 15 depict operating channels in a WLAN system;

FIG. 16 depicts a Network Allocation Vector (NAV) update operation of anSTA according to the present invention;

FIG. 17 depicts an operation of a third-party STA when a Downlink (DL)Orthogonal Frequency Division Multiple Access (OFDMA) PPDU istransmitted according to the present invention;

FIG. 18 depicts a NAV update operation of an STA according to thepresent invention;

FIG. 19 depicts an exemplary operation for transmitting anACKnowledgement (ACK) in response to DL Multi-User (MU) transmission inan Uplink (UL) Single User (SU) transmission scheme;

FIG. 20 depicts an exemplary operation for transmitting an ACK inresponse to UL MU transmission in a UL MU transmission scheme;

FIGS. 21 and 22 depict various types of UL responses to DL MUtransmission; and

FIG. 23 depicts an exemplary method according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description, only certain embodiments of thepresent invention have been shown and described, simply by way ofillustration. As those skilled in the art would realize, the describedembodiments may be modified in various different ways, all withoutdeparting from the spirit or scope of the present invention.Accordingly, the drawings and description are to be regarded asillustrative in nature and not restrictive. Like reference numeralsdesignate like elements throughout the specification.

In a Wireless Local Area network (WLAN), a Basic Service Set (BSS)includes a plurality of WLAN devices. A WLAN device may include a MediumAccess Control (MAC) layer and a PHYsical (PHY) layer according toInstitute of Electrical and Electronics Engineers (IEEE) 802.11 seriesstandards. In the plurality of WLAN devices, at least one the WLANdevice may be an Access Point (AP) and the other WLAN devices may benon-AP Stations (non-AP STAs). Alternatively, all of the plurality ofWLAN devices may be non-AP STAs in an ad-hoc networking environment. Ingeneral, AP STA and non-AP STA may be each referred to as a STA or maybe collectively referred to as STAs. However, for ease of descriptionherein, only the non-AP STAs may be referred to herein as the STAs.

FIG. 1 is a block diagram of a WLAN device.

Referring to FIG. 1, a WLAN device 1 includes a baseband processor 10, aRadio Frequency (RF) transceiver 20, an antenna unit 30, a memory 40, aninput interface unit 50, an output interface unit 60, and a bus 70.

The baseband processor 10 may be simply referred to as a processor,performs baseband signal processing described in the presentspecification, and includes a MAC processor (or MAC entity) 11 and a PHYprocessor (or PHY entity) 15.

In an embodiment of the present invention, the MAC processor 11 mayinclude a MAC software processing unit 12 and a MAC hardware processingunit 13. The memory 40 may store software (hereinafter referred to as‘MAC software’) including at least some functions of the MAC layer. TheMAC software processing unit 12 may execute the MAC software toimplement some functions of the MAC layer, and the MAC hardwareprocessing unit 13 may implement the remaining functions of the MAClayer in hardware (hereinafter referred to as ‘MAC hardware’). However,the MAC processor 11 is not limited to the foregoing implementationexamples.

The PHY processor 15 includes a transmitting (TX) signal processing unit100 and a receiving (RX) signal processing unit 200.

The baseband processor 10, the memory 40, the input interface unit 50,and the output interface unit 60 may communicate with one another viathe bus 70.

The RF transceiver 20 includes an RF transmitter 21 and an RF receiver22.

The memory 40 may further store an Operating System (OS) andapplications. The input interface unit 50 receives information from auser, and the output interface unit 60 outputs information to the user.

The antenna unit 30 includes one or more antennas. When Multiple inputMultiple Output (MIMO) or Multi-User MIMO (MU-MIMO) is used, the antennaunit 30 may include a plurality of antennas.

FIG. 2 is a schematic block diagram of an exemplary transmission signalprocessor in a WLAN.

Referring to FIG. 2, the transmitting signal processing unit 100 mayinclude an encoder 110, an interleaver 120, a mapper 130, an InverseFourier Transformer (IFT) 140, and a Guard Interval (GI) inserter 150.

The encoder 110 encodes input data. For example, the encoder 110 may bea Forward Error Correction (FEC) encoder. The FEC encoder may include aBinary Convolutional Code (BCC) encoder followed by a puncturing device,or the FEC encoder may include a Low-Density Parity-Check (LDPC)encoder.

The transmitting signal processing unit 100 may further include ascrambler for scrambling the input data before encoding to reduce theprobability of long sequences of 0s or 1s. If BCC encoding is used inthe encoder 110, the transmitting signal processing unit 100 may furtherinclude an encoder parser for demultiplexing the scrambled bits among aplurality of BCC encoders. If LDPC encoding is used in the encoder 110,the transmitting signal processing unit 100 may not use the encoderparser.

The interleaver 120 interleaves the bits of each stream output from theencoder 110 to change the order of bits. Interleaving may be appliedonly when BCC encoding is used in the encoder 110. The mapper 130 mapsthe sequence of bits output from the interleaver 120 to constellationpoints. If LDPC encoding is used in the encoder 110, the mapper 130 mayfurther perform LDPC tone mapping in addition to constellation mapping.

When MIMO or MU-MIMO is used, the transmitting signal processing unit100 may use a plurality of interleavers 120 and a plurality of mappers130 corresponding to the number of spatial streams, N_(SS). In thiscase, the transmitting signal processing unit 100 may further include astream parser for dividing outputs of the BCC encoders or output of theLDPC encoder into blocks that are sent to different interleavers 120 ormappers 130. The transmitting signal processing unit 100 may furtherinclude a Space-Time Block Code (STBC) encoder for spreading theconstellation points from the N_(SS) spatial streams into N_(STS)space-time streams and a spatial mapper for mapping the space-timestreams to transmit chains. The spatial mapper may use direct mapping,spatial expansion, or beamforming.

The IFT 140 converts a block of constellation points output from themapper 130 or the spatial mapper to a time-domain block (i.e., a symbol)by using Inverse Discrete Fourier Transform (IDFT) or Inverse FastFourier Transform (IFFT). If the STBC encoder and the spatial mapper areused, the IFT 140 may be provided for each transmit chain.

When MIMO or MU-MIMO is used, the transmitting signal processing unit100 may insert Cyclic Shift Diversities (CSDs) to prevent unintentionalbeamforming. The CSD insertion may occur before or after IFT. The CSDmay be specified per transmit chain or may be specified per space-timestream. Alternatively, the CSD may be applied as a part of the spatialmapper.

When MU-MIMO is used, some blocks before the spatial mapper may beprovided for each user.

The GI inserter 150 prepends a GI to the symbol. The transmitting signalprocessing unit 100 may optionally perform windowing to smooth edges ofeach symbol after inserting the GI. The RF transmitter 21 converts thesymbols into an RF signal and transmits the RF signal via the antennaunit 30. When MIMO or MU-MIMO is used, the GI inserter 150 and the RFtransmitter 21 may be provided for each transmit chain.

FIG. 3 is a schematic block diagram of an exemplary a receiving signalprocessor in a WLAN.

Referring to FIG. 3, the receiving signal processing unit 200 includes aGI remover 220, a Fourier Transformer (FT) 230, a demapper 240, adeinterleaver 250, and a decoder 260.

An RF receiver 22 receives an RF signal via the antenna unit 30 andconverts the RF signal into symbols. The GI remover 220 removes the GIfrom the symbol. When MIMO or MU-MIMO is used, the RF receiver 22 andthe GI remover 220 may be provided for each receive chain.

The FT 230 converts the symbol (i.e., the time-domain block) into ablock of constellation points by using a Discrete Fourier Transform(DFT) or a Fast Fourier Transform (FFT). The FT 230 may be provided foreach receive chain.

When MIMO or MU-MIMO is used, the receiving signal processing unit 200may include a spatial demapper for converting Fourier Transformedreceiver chains to constellation points of the space-time streams, andan STBC decoder for despreading the constellation points from thespace-time streams into the spatial streams.

The demapper 240 demaps the constellation points output from the FT 230or the STBC decoder to bit streams. If LDPC encoding is applied to thereceived signal, the demapper 240 may further perform LDPC tonedemapping before constellation demapping. The deinterleaver 250deinterleaves the bits of each stream output from the demapper 240.Deinterleaving may be applied only when a BCC encoding scheme is appliedto the received signal.

When MIMO or MU-MIMO is used, the receiving signal processing unit 200may use a plurality of demappers 240 and a plurality of deinterleavers250 corresponding to the number of spatial streams. In this case, thereceiving signal processing unit 200 may further include a streamdeparser for combining streams output from the deinterleavers 250.

The decoder 260 decodes the streams output from the deinterleaver 250 orthe stream deparser. For example, the decoder 100 may be an FEC decoder.The FEC decoder may include a BCC decoder or an LDPC decoder. Thereceiving signal processing unit 200 may further include a descramblerfor descrambling the decoded data. If BCC decoding is used in thedecoder 260, the receiving signal processing unit 200 may furtherinclude an encoder deparser for multiplexing the data decoded by aplurality of BCC decoders. If LDPC decoding is used in the decoder 260,the receiving signal processing unit 200 may not use the encoderdeparser.

In a WLAN system, Carrier Sense Multiple Access with Collision Avoidance(CSMA/CA) is a basic MAC access mechanism. The CSMA/CA mechanism isreferred to as Distributed Coordination Function (DCF) of IEEE 802.11MAC, shortly as a ‘listen before talk’ access mechanism. According tothe CSMA/CA mechanism, an AP and/or a STA may sense a medium or achannel for a predetermined time before starting transmission, that is,may perform Clear Channel Assessment (CCA). If the AP or the STAdetermines that the medium or channel is idle, it may start to transmita frame on the medium or channel. On the other hand, if the AP and/orthe STA determines that the medium or channel is occupied or busy, itmay set a delay period (e.g., a random backoff period), wait for thedelay period without starting transmission, and then attempt to transmita frame. By applying a random backoff period, a plurality of STAs areexpected to attempt frame transmission after waiting for different timeperiods, resulting in minimizing collisions.

FIG. 4 depicts a relationship between InterFrame Spaces (IFSs).

WLAN devices may exchange data frames, control frames, and managementframes with each other.

A data frame is used for transmission of data forwarded to a higherlayer. The WLAN device transmits the data frame after performing backoffif a Distributed Coordination Function IFS (DIFS) has elapsed from atime when the medium has been idle. A management frame is used forexchanging management information which is not forwarded to the higherlayer. The WLAN device transmits the management frame after performingbackoff if an IFS such as the DIFS or a Point Coordination Function IFS(PIFS) has elapsed. Subtype frames of the management frame include abeacon frame, an association request/response frame, a proberequest/response frame, and an authentication request/response frame. Acontrol frame is used for controlling access to the medium. Subtypeframes of the control frame include a Request-To-Send (RTS) frame, aClear-To-Send (CTS) frame, and an ACKnowledgement (ACK) frame. In thecase that the control frame is not a response frame to another frame,the WLAN device transmits the control frame after performing backoff ifthe DIFS has elapsed. In case that the control frame is a response frameto another frame, the WLAN device transmits the control frame withoutperforming backoff if a Short IFS (SIFS) has elapsed. The type andsubtype of a frame may be identified by a type field and a subtype fieldin a Frame Control (FC) field.

On the other hand, a Quality of Service (QoS) STA transmits a frameafter performing backoff if an Arbitration IFS (AIFS) for an associatedAccess Category (AC), i.e., AIFS[i] (i is determined based on AC) haselapsed. In this case, the AIFC[i] may be used for a data frame, amanagement frame, or a control frame that is not a response frame.

In the example illustrated in FIG. 4, upon generation of a frame to betransmitted, a STA may transmit the frame immediately, if it determinesthat the medium is idle for the DIFS or AIFS[i] or longer. The medium isbusy for a time period during which the STA transmits the frame. Duringthe time period, upon generation of a frame to be transmitted, anotherSTA may defer access by confirming that the medium is busy. If themedium gets idle, the STA that intends to transmit the frame may performa backoff operation after a predetermined IFS in order to minimizecollision with any other STA. Specifically, the STA that intends totransmit the frame selects a random backoff count, waits for a slot timecorresponding to the selected random backoff count, and then attempttransmission. The random backoff count is determined based on aContention Window (CW) parameter and the medium is monitoredcontinuously during count-down of backoff slots (i.e. decrement abackoff count-down) according to the determined backoff count. If theSTA monitors the medium as busy, the STA discontinues the count-down andwaits, and then, if the medium gets idle, the STA resumes thecount-down. If the backoff slot count reaches 0, the STA may transmitthe next frame.

FIG. 5 is a conceptual diagram illustrating a CSMA/CA-based frametransmission procedure for avoiding collisions between frames in achannel.

Referring FIG. 5, a first STA (STA1) is a transmit WLAN device fortransmitting data, a second STA (STA2) is a receive WLAN device forreceiving the data from STA1, and a third STA (STA3) is a WLAN devicewhich may be located in an area where a frame transmitted from STA1and/or a frame transmitted from STA2 can be received by STA3.

STA1 may determine whether the channel is busy by carrier sensing. TheSTA1 may determine the channel occupation based on an energy level onthe channel or correlation of signals in the channel, or may determinethe channel occupation by using a Network Allocation Vector (NAV) timer.

After determining that the channel is not being used by other devicesduring DIFS (that is, the channel is idle), STA1 may transmit an RTSframe to STA2 after performing backoff. Upon receiving the RTS frame,STA2 may transmit a CTS frame as a response to the CTS frame after SIFS.

When STA3 receives the RTS frame, STA3 may set the NAV timer for atransmission duration of subsequently transmitted frame by usingduration information included in the RTS frame. For example, the NAVtimer may be set for a duration of SIFS+CTS frame duration+SIFS+dataframe duration+SIFS+ACK frame duration. When STA3 receives the CTSframe, it may set the NAV timer for a transmission duration ofsubsequently transmitted frames by using duration information includedin the CTS frame. For example, the NAV timer may be set for a durationof SIFS+a data frame duration+SIFS+an ACK frame duration. Upon receivinga new frame before the NAV timer expires, STA3 may update the NAV timerby using duration information included in the new frame. STA3 does notattempt to access the channel until the NAV timer expires.

When STA1 receives the CTS frame from STA2, it may transmit a data frameto STA2 after SIFS elapsed from the CTS frame has been completelyreceived. Upon successfully receiving the data frame, STA2 may transmitan ACK frame as a response to the data frame after SIFS elapsed.

When the NAV timer expires, STA3 may determine whether the channel isbusy through the use of carrier sensing. Upon determining that thechannel is not in use by other devices during DIFS and after the NAVtimer has expired, STA3 may attempt channel access after a contentionwindow after a random backoff has elapsed.

FIG. 6 depicts an exemplary frame structure in a WLAN system.

PHY layer may prepare a transmission MAC PDU (MPDU) in response to aninstruction (or a primitive, which is a set of instructions or a set ofparameters) by the MAC layer. For example, upon receipt of aninstruction requesting transmission start from the MAC layer, the PHYlayer may switch to a transmission mode, construct a frame withinformation (e.g., data) received from the MAC layer, and transmit theframe.

Upon detection of a valid preamble in a received frame, the PHY layermonitors a header of the preamble and transmits an instructionindicating reception start of the PHY layer to the MAC layer.

Information is transmitted and received in frames in the WLAN system.For this purpose, a Physical layer Protocol Data Unit (PPDU) frameformat is defined.

A PPDU frame may include a Short Training Field (STF) field, a LongTraining Field (LTF) field, a SIGNAL (SIG) field, and a Data field. Themost basic (e.g., a non-High Throughput (non-HT)) PPDU frame may includeonly a Legacy-STF (L-STF) field, a Legacy-LTF (L-LTF) field, a SIGfield, and a Data field. Additional (or other types of) STF, LTF, andSIG fields may be included between the SIG field and the Data fieldaccording to the type of a PPDU frame format (e.g., an HT-mixed formatPPDU, an HT-greenfield format PPDU, a Very High Throughput (VHT) PPDU,etc.).

The STF is used for signal detection, Automatic Gain Control (AGC),diversity selection, fine time synchronization, etc. The LTF field isused for channel estimation, frequency error estimation, etc. The STFand the LTF fields may be referred to as signals for OFDM PHY layersynchronization and channel estimation.

The SIG field may include a RATE field and a LENGTH field. The RATEfield may include information about a modulation scheme and coding rateof data. The LENGTH field may include information about the length ofthe data. The SIG field may further include parity bits, SIG TAIL bits,etc.

The Data field may include a SERVICE field, a Physical layer ServiceData Unit (PSDU), and PPDU TAIL bits. When needed, the Data field mayfurther include padding bits. A part of the bits of the SERVICE fieldmay be used for synchronization at a descrambler of a receiver. The PSDUcorresponds to a MAC PDU defined at the MAC layer and may include datagenerated/used in a higher layer. The PPDU TAIL bits may be used toreturn an encoder to a zero state. The padding bits may be used to matchthe length of the Data filed in predetermined units.

A MAC PDU is defined according to various MAC frame formats. A basic MACframe includes a MAC header, a frame body, and a Frame Check Sequence(FCS). The MAC frame includes a MAC PDU and may be transmitted andreceived in the PSDU of the data part in the PPDU frame format.

The MAC header includes a Frame Control field, a Duration/Identifier(ID) field, an Address field, etc. The Frame Control field may includecontrol information required for frame transmission/reception. TheDuration/ID field may be set to a time for transmitting the frame. Fordetails of Sequence Control, QoS Control, and HT Control subfields ofthe MAC header, refer to the IEEE 802.11-2012 technical specification.

The Frame Control field of the MAC header may include Protocol Version,Type, Subtype, To DS, From DS, More Fragment, Retry, Power Management,More Data, Protected Frame, and Order subfields. For the contents ofeach subfield in the Frame Control field, refer to the IEEE 802.11-2012technical specification.

A Null-Data Packet (NDP) frame format is a frame format that does notinclude a data packet. In other words, the NDP frame format includesonly a Physical Layer Convergence Protocol (PLCP) header part (i.e., theSTF, LTF, and SIG fields) of the general PPDU frame format, without theremaining part (i.e., the Data field) of the general PPDU frame format.The NDP frame format may be referred to as a short frame format.

The IEEE 802.11 ax task group is discussing a WLAN system, called a HighEfficiency WLAN (HEW) system, that operates in 2.4 GHz or 5 GHz andsupports a channel bandwidth (or channel width) of 20 MHz, 40 MHz, 80MHz, or 160 MHz. The present invention defines a new PPDU frame formatfor the IEEE 802.11ax HEW system. The new PPDU frame format may supportMU-MIMO or OFDMA. A PPDU of the new format may be referred to as a ‘HEWPPDU’ or ‘HE PPDU’ (similarly, HEW xyz may be referred to as ‘HE xyz’ or‘HE-xyz’ in the following descriptions).

In present specification, the term ‘MU-MIMO or OFDMA mode’ includesMU-MIMO without using OFDMA, or OFDMA mode without using MU-MIMO in anorthogonal frequency resource, or OFDMA mode using MU-MIMO in anorthogonal frequency resource.

FIG. 7 depicts an exemplary HE PPDU frame format.

A transmitting STA may generate a PPDU frame according to the HE PPDUframe format as illustrated in FIG. 7 and transmit the PPDU frame to areceiving STA. The receiving STA may detect a PPDU and then process thePPDU.

The HE PPDU frame format may broadly include two parts: the first partincluding an L-STF field, an L-LTF field, an L-SIG field, an RL-SIGfield, a HE-SIG-A field, and a HE-SIG-B field and the second partincluding a HE-STF field, a HE-LTF field, and a HE-DATA field. 64-FFTbased on a channel bandwidth of 20 MHz may be applied to the first partand a basic subcarrier spacing of 312.5 kHz and a basic DFT period of3.2 μs may be included in the first part. 256-FFT based on a channelbandwidth of 20 MHz may be applied to the second part and a basicsubcarrier spacing of 75.125 kHz and a basic DFT period of 12.8 μs maybe included in the second part.

The HE-SIG-A field may include N_(HESIGA) symbols, the HE-SIG-B fieldmay include N_(HESIGB) symbols, the HE-LTF field may include N_(HELTF)symbols, and the HE-DATA field may include N_(DATA) symbols.

A detailed description of the fields included in the HE PPDU frameformat is given in Table 1.

TABLE 1 DFT Subcarrier Element definition duration period GI spacingDescription Legacy(L)- Non-high 8 μs — — equivalent to L-STF of anon-trigger based STF throughput(HT) 1,250 kHz PPDU has a periodicity of0.8 μs Short Training with 10 periods. field L-LTF Non-HT Long 8 μs  3.2μs 1.6 μs  312.5 kHz Training field L-SIG Non-HT 4 μs  3.2 μs 0.8 μs 312.5 kHz SIGNAL field RL-SIG Repeated Non- 4 μs  3.2 μs 0.8 μs  312.5kHz HT SIGNAL field HE-SIG-A HE SIGNAL A N_(HESIGA)  3.2 μs 0.8 μs 312.5 kHz HE-SIG-A is duplicate on each field * 4 μs 20 MHz segmentafter the legacy preamble to indicate common control information.N_(HESIGA) means the number of OFDM symbols of the HE-SIG-A field and isequal to 2 or 4. HE-SIG-B HE SIGNAL B N_(HESIGB)  3.2 μs 0.8 μs  312.5kHz N_(HESIGB) means the number of field * 4 μs OFDM symbols of theHE-SIG-B field and is variable. DL MU packet contains HE-SIG-B. SUpackets and UL Trigger based packets do not contain HE-SIG-B. HE-STF HEShort 4 or 8 μs — — non- HE-STF of a non-trigger-based Training fieldTrigger- PPDU has a periodicity of 0.8 μs based with 5 periods. Anon-trigger-based PPDU: PPDU is not sent in response to a (equivalenttrigger frame. to) 1,250 The HE-STF of a trigger-based kHz; PPDU has aperiodicity of 1.6 μs trigger- with 5 periods. A trigger-based basedPPDU is an UL PPDU sent in PPDU: response to a trigger frame.(equivalent to) 625 kHz HE-LTF HE Long N_(HELTF) * 2xLTF: supports2xLTF: HE PPDU shall support 2xLTF Training field (DTF 6.4 μs 0.8, 1.6,(equivalent mode and 4xLTF mode. period + 4xLTF: 3.2 μs to) 156.25 Inthe 2xLTF mode, HE-LTF GI) μs 12.8 μs kHz; symbol excluding GI isequivalent 4xLTF: to modulating every other tone in 78.125 kHz an OFDMsymbol of 12.8 μs excluding GI, and then removing the second half of theOFDM symbold in time domain. N_(HELTF) means the number of HE-LTFsymbols and is equal to 1, 2, 4, 6, 8. HE-DATA HE DATA N_(DATA) * 12.8μs supports 78.125 kHz N_(DATA) measn the number of HE field (DTF 0.8,1.6, data symbols. period + 3.2 μs GI) μs

L-STF is a non-HT Short Training field and may have a duration of 8 μsand a subcarrier spacing equivalent to 1250 kHz. L-STF of a PPDU whichis not based on a trigger may have a periodicity of 0.8 μs with 10periods. Herein, the trigger corresponds to scheduling information forUL transmission.

L-LTF is a non-HT Long Training field and may have a duration of 8 μs, aDFT period of 3.2 μs, a Guard Interval (GI) of 1.6 μs, and a subcarrierspacing of 312.5 kHz.

L-SIG is a non-HT SIGNAL field and may have a duration of 4 μs, a DFTperiod of 3.2 μs, a GI of 0.8 μs, and a subcarrier spacing of 312.5 kHz.

RL-SIG is a Repeated Non-HT SIGNAL field and may have a duration of 4μs, a DFT period of 3.2 μs, a GI of 0.8 μs, and a subcarrier spacing of312.5 kHz.

L-STF, L-LTF, L-SIG, and RL-SIG may be called legacy preambles.

HE-SIG-A is a HE SIGNAL A field and may have a duration of N_(HESIGA)*4μs, a DFT period of 3.2 μs, a GI of 0.8 μs, and a subcarrier spacing of312.5 kHz. HE-SIG-A may be duplicated on each 20 MHz segment after thelegacy preambles to indicate common control information. N_(HESIGA)represents the number of OFDM symbols of the HE-SIG-A field and may havea value of 2 or 4.

HE-SIG-B is a HE SIGNAL B field and may have a duration of N_(HESIGB)*4μs, a DFT period of 3.2 μs, a GI of 0.8 μs, and a subcarrier spacing of312.5 kHz. N_(HESIGB) represents the number of OFDM symbols of theHE-SIG-B field and may have a variable value. In addition, although a DLMulti-User (MU) packet may include the HE-SIG-B field, a Single-User(SU) packet and a UL trigger based packet may not include the HE-SIG-Bfield.

HE-STF is a HE Short Training field and may have a duration of 4 or 8μs. A non-trigger based PPDU may have a subcarrier spacing equivalent to1250 kHz and a trigger based PPDU may have a subcarrier spacingequivalent to 625 kHz. HE-STF of the non-triggered PPDU may have aperiodicity of 0.8 μs with 4 periods. The non-triggered PPDU is nottransmitted in response to a trigger field. HE-STF of the trigger basedPPDU may have a periodicity of 1.6 μs with 5 periods. The trigger basedPPDU is a UL PPDU transmitted in response to the trigger frame.

HE-LTF is a HE Long Training field and may have a duration ofN_(HELTF)*(DFT period+GI)μs. N_(HELTF) represents the number of HE-LTFsymbols and may have a value of 1, 2, 4, 6, or 8. A HE PPDU may supporta 2×LTF mode and a 4×LTF mode. In the 2×LTF mode, a HE-LTF symbol exceptfor a GI is equivalent to a symbol obtained by modulating every othertone in an OFDM symbol of 12.8 μs excluding a GI and then eliminatingthe first half or the second half of the OFDM symbol in the time domain.In the 4×LTF mode, a HE-LTF symbol excluding a GI are equivalent to asymbol obtained by modulating every fourth tone in an OFDM symbol of12.8 μs excluding a GI and then eliminating the first three-fourths orthe last three-fourths of the OFDM symbol in the time domain. 2×LTF mayhave a DFT period of 6.4 μs and 4×LTF may have a DFT period of 12.8 μs.A GI of HE-LTF may support 0.8 μs, 1.6 μs, and 3.2 μs. 2×LTF may have asubcarrier spacing equivalent to 156.25 kHz and 4×LTF may have asubcarrier spacing of 78.125 kHz.

HE-DATA is a HE DATA field and may have a duration of, N_(DATA)*(DFTperiod+GI)μs. N_(DATA) represents the number of HE-DATA symbols. HE-DATAmay have a DFT period of 12.8 μs. A GI of HE-DATA may support 0.8 μs,1.6 μs, and 3.2 μs. HE-DATA may have a subcarrier spacing of 78.125 kHz.

The above description of the fields included in the HE PPDU frame formatmay be combined with exemplary HE PPDU frame formats described below.For example, characteristics of fields exemplarily described below maybe applied while a transmission order of the fields of the HE PPDU frameformat of FIG. 7 is maintained.

FIG. 8 depicts an exemplary HE PPDU frame format according to thepresent invention.

Referring to FIG. 8, the vertical axis represents frequency and thehorizontal axis represents time. It is assumed that frequency and timeincrease in the upward direction and the right direction, respectively.

In the example of FIG. 8, one channel includes four subchannels. AnL-STF, an L-LTF, an L-SIG, and an HE-SIG-A may be transmitted perchannel (e.g., 20 MHz), a HE-STF and a HE-LTF may be transmitted on eachsubchannel being a basic subchannel unit (e.g., 5 MHz), and a HE-SIG-Band a PSDU may be transmitted on each of subchannels allocated to a STA.A subchannel allocated to a STA may have a size required for PSDUtransmission to the STA. The size of the subchannel allocated to the STAmay be N (N=1, 2, 3, . . . ) times as large as the size of basicsubchannel unit (i.e., a subchannel having a minimum size). In theexample of FIG. 8, the size of a subchannel allocated to each STA isequal to the size of the basic subchannel unit. For example, a firstsubchannel may be allocated for PSDU transmission from an AP to STA1 andSTA2, a second subchannel may be allocated for PSDU transmission fromthe AP to STA3 and STA4, a third subchannel may be allocated for PSDUtransmission from the AP to STA5, and a fourth subchannel may beallocated for PSDU transmission from the AP to STA6.

While the term subchannel is used in the present disclosure, the termsubchannel may be referred to as Resource Unit (RU) or subband. Inparticular, the terms like OFDMA subchannel, OFDMA RU, OFDMA subband canbe used in embodiments for OFDMA in the present disclosure. Terms like abandwidth of a subchannel, a number of tones (or subcarriers) allocatedto a subchannel, a number of data tones (or data subcarriers) allocatedto a subchannel can be used to express a size of a subchannel. Asubchannel refers to a frequency band allocated to a STA and a basicsubchannel unit refers to a basic unit used to represent the size of asubchannel. While the size of the basic subchannel unit is 5 MHz in theabove example, this is purely exemplary. Thus, the basic subchannel unitmay have a size of 2.5 MHz.

In FIG. 8, a plurality of HE-LTF elements are distinguished in the timeand frequency domains. One HE-LTF element may correspond to one OFDMsymbol in time domain and one subchannel unit (i.e., a subchannelbandwidth allocated to a STA) in frequency domain. The HE-LTF elementsare logical units, and the PHY layer does not necessarily operate inunits of an HE-LTF element. In the following description, a HE-LTFelement may be referred to shortly as a HE-LTF.

A HE-LTF symbol may correspond to a set of HE-LTF elements in one OFDMsymbol in time domain and in one channel unit (e.g., 20 MHz) infrequency domain.

A HE-LTF section may correspond to a set of HE-LTF elements in one ormore OFDM symbols in time domain and in one subchannel unit (i.e., asubchannel bandwidth allocated to a STA) in frequency domain.

A HE-LTF field may be a set of HE-LTF elements, HE-LTF symbols, orHE-LTF sections for a plurality of stations.

The L-STF field is used for frequency offset estimation and phase offsetestimation, for preamble decoding at a legacy STA (i.e., a STA operatingin a system such as IEEE 802.11a/b/g/n/ac). The L-LTF field is used forchannel estimation, for the preamble decoding at the legacy STA. TheL-SIG field is used for the preamble decoding at the legacy STA andprovides a protection function for PPDU transmission of a third-partySTA (e.g., a third-party STA is not allowed to transmit during a certainperiod based on the value of a LENGTH field included in the L-SIGfield).

HE-SIG-A (or HEW SIG-A) represents High Efficiency Signal A (or HighEfficiency WLAN Signal A), and includes HE PPDU (or HEW PPDU) modulationparameters, etc. for HE preamble (or HEW preamble) decoding at a HE STA(or HEW STA). The parameters set included in the HEW SIG-A field mayinclude one or more of Very High Throughput (VHT) PPDU modulationparameters transmitted by IEEE 802.11 ac stations, as listed in [Table2] below, to ensure backward compatibility with legacy STAs (e.g., IEEE802.11ac stations).

TABLE 2 Two parts of Number VHT-SIG-A Bit Field of bits DescriptionVHT-SIG-A1 B0-B1 BW 2 Set to 0 for 20 MHz, 1 for 40 MHz, 2 for 80 MHz,and 3 for 160 MHz and 80 + 80 MHz B2 Reserved 1 Reserved. Set to 1. B3STBC 1 For a VHT SU PPDU: Set to 1 if space time block coding is usedand set to 0 otherwise. For a VHT MU PPDU: Set to 0. B4-B9 Group ID 6Set to the value of the TXVECTOR parameter GROUP_ID. A value of 0 or 63indicates a VHT SU PPDU; otherwise, indicates a VHT MU PPDU. B10-B21NSTS/Partial 12 For a VHT MU PPDU: NSTS is divided into 4 user AIDpositions of 3 bits each. User position p, where 0 ≦ p ≦ 3, uses bitsB(10 + 3p) to B(12 + 3p). The number of space- time streams for user uare indicated at user position p = USER_POSITION[u] where u = 0, 1, . .. , NUM_USERS − 1 and the notation A[b] denotes the value of array A atindex b. Zero space-time streams are indicated at positions not listedin the USER_POSITION array. Each user position is set as follows: Set to0 for 0 space-time streams Set to 1 for 1 space-time stream Set to 2 for2 space-time streams Set to 3 for 3 space-time streams Set to 4 for 4space-time streams Values 5-7 are reserved For a VHT SU PPDU: B10-B12Set to 0 for 1 space-time stream Set to 1 for 2 space-time streams Setto 2 for 3 space-time streams Set to 3 for 4 space-time streams Set to 4for 5 space-time streams Set to 5 for 6 space-time streams Set to 6 for7 space-time streams Set to 7 for 8 space-time streams B13-B21 PartialAID: Set to the value of the TXVECTOR parameter PARTIAL_AID. Partial AIDprovides an abbreviated indication of the intended recipient(s) of thePSDU (see 9.17a). B22 TXOP_PS_NOT_ALLOWED 1 Set to 0 by VHT AP if itallows non-AP VHT STAs in TXOP power save mode to enter Doze stateduring a TXOP. Set to 1 otherwise. The bit is reserved and set to 1 inVHT PPDUs transmitted by a non-AP VHT STA. B23 Reserved 1 Set to 1VHT-SIG-A2 B0 Short G1 1 Set to 0 if short guard interval is not used inthe Data field. Set to 1 if short guard interval is used in the Datafield. B1 Short G1 1 Set to 1 if short guard interval is used andN_(SYM) mod 10 = 9; N_(SYM) otherwise, set to 0. N_(SYM) is defined in22.4.3. Disambiguation B2 SU/MU[0] 1 For a VHT SU PPDU, B2 is set to 0for BCC, 1 for LDPC Coding For a VHT MU PPDU, if the MU[0] NSTS field isnonzero, then B2 indicates the coding used for user u withUSER_POSITION[u] = 0; set to 0 for BCC and 1 for LDPC. If the MU[0] NSTSfield is 0, then this field is reserved and set to 1. B3 LDPC Extra 1Set to 1 if the LDPC PPDU encoding process (if an SU OFDM PPDU), or atleast one LDPC user's PPDU encoding process Symbol (if a VHT MU PPDU),results in an extra OFDM symbol (or symbols) as described in 22.3.10.5.4and 22.3.10.5.5. Set to 0 otherwise. B4-B7 SU VHT- 4 For a VHT SU PPDU:MCS/MU[1-3] VHT-MCS index Coding For a VHT MU PPDU: If the MU[1] NSTSfield is nonzero, then B4 indicates coding for user u withUSER_POSITION[u] = 1: set to 0 for BCC, 1 for LDPC. If the MU[1] NSTSfield is 0, then B4 is reserved and set to 1. If the MU[2] NSTS field isnonzero, then B5 indicates coding for user u with USER_POSITION[u] = 2:set to 0 for BCC, 1 for LDPC. If the MU[2] NSTS field is 0, then B5 isreserved and set to 1. If the MU[3] NSTS field is nonzero, then B6indicates coding for user u with USER_POSITION[u] = 3: set to 0 for BCC,1 for LDPC. If the MU[3] NSTS field is 0, then B6 is reserved and setto 1. B7 is reserved and set to 1 B8 Beamformed 1 For a VHT SU PPDU: Setto 1 if a Beamforming steering matrix is applied to the waveform in anSU transmission as described in 20.3.11.11.2, set to 0 otherwise. For aVHT MU PPDU: Reserved and set to 1 NOTE - If equal to 1 smoothing is notrecommended. B9 Reserved 1 Reserved and set to 1 B10-B17 CRC 8 CRCcalculated as in 20.3.9.4.4 with c7 in B10. Bits 0-23 of HT-SIG1 andbits 0-9 of HT-SIG2 are replaced by bits 0.23 of VHT-SIG-A1 and bits 0-9of VHT-SIG-A2, respectively. B18-B23 Tail 6 Used to terminate thetrellis of the convolutional decoder. Set to 0.

[Table 2] illustrates fields, bit positions, numbers of bits, anddescriptions included in each of two parts, VHT-SIG-A1 and VHT-SIG-A2,of the VHT-SIG-A field defined by the IEEE 802.11ac standard. Forexample, a BW (BandWidth) field occupies two Least Significant Bits(LSBs), B0 and B1 of the VHT-SIG-A1 field and has a size of 2 bits. Ifthe 2 bits are set to 0, 1, 2, or 3, the BW field indicates 20 MHz, 40MHz, 80 MHz, or 160 and 80+80 MHz. For details of the fields included inthe VHT-SIG-A field, refer to the IEEE 802.11ac-2013 technicalspecification. In the HE PPDU frame format of the present invention, theHE-SIG-A field may include one or more of the fields included in theVHT-SIG-A field, and it may provide backward compatibility with IEEE802.11ac stations.

FIG. 9 depicts subchannel allocation in the HE PPDU frame formataccording to the present invention.

In FIG. 9, it is assumed that information indicating subchannelsallocated to STAs in HE PPDU indicates that 0 MHz subchannel isallocated to STA1 (i.e., no subchannel is allocated), a 5-MHz subchannelis allocated to each of STA2 and STA3, and a 10-MHz subchannel isallocated to STA4.

In the example of FIG. 9, an L-STF, an L-LTF, an L-SIG, and a HE-SIG-Amay be transmitted per channel (e.g., 20 MHz), a HE-STF and a HE-LTF maybe transmitted on each of subchannels being basic subchannel units(e.g., 5 MHz), and a HE-SIG-B and a PSDU may be transmitted on each ofsubchannels allocated to STAs. A subchannel allocated to a STA has asize required for PSDU transmission to the STA. The size of thesubchannel allocated to the STA may be an N (N=1, 2, 3, . . . ) multipleof the size of the basic subchannel unit (i.e., a minimum-sizesubchannel unit). In the example of FIG. 9, the size of a subchannelallocated to STA2 is equal to that of the basic subchannel unit, thesize of a subchannel allocated to STA3 is equal to that of the basicsubchannel unit, and the size of a subchannel allocated to STA4 is twicelarger than that of the basic subchannel unit.

FIG. 9 illustrates a plurality of HE-LTF elements and a plurality ofHE-LTF subelements which are distinguished in the time and frequencydomains. One HE-LTF element may correspond to one OFDM symbol in thetime domain and one subchannel unit (i.e., the bandwidth of a subchannelallocated to a STA) in the frequency domain. One HE-LTF subelement maycorrespond to one OFDM symbol in the time domain and one basicsubchannel unit (e.g. 5 MHz) in the frequency domain. In the example ofFIG. 9, one HE-LTF element includes one HE-LTF subelement in the 5-MHzsubchannel allocated to STA2 or STA3. On the other hand, one HE-LTFelement includes two HE-LTF subelements in the third subchannel, i.e.,10-MHz subchannel, allocated to STA4. A HE-LTF element and a HE-LTFsubelement are logical units and the PHY layer does not always operatein units of a HE-LTF element or HE-LTF subelement.

A HE-LTF symbol may correspond to a set of HE-LTF elements in one OFDMsymbol in the time domain and one channel unit (e.g. 20 MHz) in thefrequency domain. That is, one HE-LTF symbol may be divided into HE-LTFelements by a subchannel width allocated to a STA and into HE-LTFsubelements by the width of the basic subchannel unit in the frequencydomain.

A HE-LTF section may correspond to a set of HE-LTF elements in one ormore OFDM symbols in the time domain and one subchannel unit (i.e. thebandwidth of a subchannel allocated to a STA) in the frequency domain. AHE-LTF subsection may correspond to a set of HE-LTF elements in one ormore OFDM symbols in the time domain and one basic subchannel unit(e.g., 5 MHz) in the frequency domain. In the example of FIG. 9, oneHE-LTF section includes one HE-LTF subsection in the 5-MHz subchannelallocated to STA2 or STA3. On the other hand, one HE-LTF sectionincludes two HE-LTF subsections in the third subchannel, i.e., 10-MHzsubchannel, allocated to STA4.

A HE-LTF field may correspond to a set of HE-LTF elements (orsubelements), HE-LTF symbols, or HE-LTF sections (or subsections) for aplurality of stations.

For the afore-described HE PPDU transmission, subchannels allocated to aplurality of HE STAs may be contiguous in the frequency domain. In otherwords, for HE PPDU transmission, the subchannels allocated to the HESTAs may be sequential and any intermediate one of the subchannels ofone channel (e.g., 20 MHz) may not be allowed to be unallocated orempty. Referring to FIG. 8, if one channel includes four subchannels, itmay not be allowed to keep the third subchannel unallocated and empty,while the first, second, and fourth subchannels are allocated to STAs.However, the present invention does not exclude non-allocation of anintermediate subchannel of one channel to a STA.

FIG. 10 depicts a subchannel allocation method according to the presentinvention.

In the example of FIG. 10, a plurality of contiguous channels (e.g.,20-MHz-bandwidth channels) and boundaries of the plurality of contiguouschannels are shown. In FIG. 10, a preamble may correspond to an L-STF,an L-LTF, an L-SIG, and a HE-SIG-A as illustrated in the examples ofFIGS. 8 and 9.

A subchannel for each HE STA may be allocated only within one channel,and may not be allocated with partially overlapping between a pluralityof channels. That is, if there are two contiguous 20-MHz channels CH1and CH2, subchannels for STAs paired for MU-MIMO-mode or OFDMA-modetransmission may be allocated either within CH1 or within CH2, and itmay be prohibited that one part of a subchannel exists in CH1 andanother part of the subchannel exists in CH2. This means that onesubchannel may not be allocated with crossing a channel boundary. Fromthe perspective of RUs supporting the MU-MIMO or OFDMA mode, a bandwidthof 20 MHz may be divided into one or more RUs, and a bandwidth of 40 MHzmay be divided into one or more RUs in each of two contiguous 20-MHzbandwidths, and no RU is allocated with crossing the boundary betweentwo contiguous 20-MHz bandwidths.

As described above, it is not allowed that one subchannel belongs to twoor more 20-MHz channels. Particularly, a 2.4-GHz OFDMA mode may supporta 20-MHz OFDMA mode and a 40-MHz OFDMA mode. In the 2.4-GHz OFDMA mode,it may not be allowed that one subchannel belongs to two or more 20-MHzchannels.

FIG. 10 is based on the assumption that subchannels each having the sizeof a basic subchannel unit (e.g., 5 MHz) in CH1 and CH2 are allocated toSTA1 to STA7, and subchannels each having double the size (e.g., 10 MHz)of the basic subchannel unit in CH4 and CH5 are allocated to STA8, STA9,and STA10.

As illustrated in the lower part of FIG. 9, although a subchannelallocated to STA1, STA2, STA3, STA5, STA6, or STA7 is fully overlappedonly with one channel (i.e., without crossing the channel boundary, orbelonging only to one channel), a subchannel allocated to STA4 ispartially overlapped with the two channels (i.e., crossing the channelboundary, or belonging to the two channels). In the forgoing example ofthe present invention, the subchannel allocation to STA4 is not allowed.

As illustrated in the upper part of FIG. 9, although a subchannelallocated to STA8 or STA10 is fully overlapped only with one channel(i.e., crossing the channel boundary, or belonging only to one channel),a subchannel allocated to STA9 is partially overlapped with two channels(i.e., crossing the channel boundary, or belonging to the two channels).In the forgoing example of the present invention, the subchannelallocation to STA9 is not allowed.

On the other hand, it may be allowed to allocate a subchannel partiallyoverlapped between a plurality of channels (i.e., crossing the channelboundary, or belonging to two channels). For example, in SU-MIMO modetransmission, a plurality of contiguous channels may be allocated to aSTA and any of one or more subchannels allocated to the STA may crossthe boundary between two contiguous channels.

While the following description is given with an assumption that onesubchannel has a channel bandwidth of 5 MHz in one channel having achannel bandwidth of 20 MHz, this is provided to simplify thedescription of the principle of the present invention and thus shouldnot be construed as limiting the present invention. For example, thebandwidths of a channel and a subchannel may be defined or allocated asvalues other than the above examples. In addition, a plurality ofsubchannels in one channel may have the same or different channelwidths.

FIG. 11 depicts the starting and ending points of a HE-LTF field in theHE PPDU frame format according to the present invention.

To support the MU-MIMO mode and the OFDMA mode, the HE PPDU frame formataccording to the present invention may include, in the HE-SIG-A field,information about the number of spatial streams to be transmitted to aHE STA allocated to each subchannel.

If MU-MIMO-mode or OFDMA-mode transmission is performed to a pluralityof HE STAs on one subchannel, the number of spatial streams to betransmitted to each of the HE STAs may be provided in the HE-SIG-A orHE-SIG-B field, which will be described later in detail.

FIG. 11 is based on the assumption that a first 5-MHz subchannel isallocated to STA1 and STA2 and two spatial streams are transmitted toeach STA in a DL MU-MIMO or OFDMA mode (i.e., a total of four spatialstreams are transmitted on one subchannel). For this purpose, a HE-STF,a HE-LTF, a HE-LTF, a HE-LTF, a HE-LTF, and a HE-SIG-B follow theHE-SIG-A field on the subchannel. The HE-STF is used for frequencyoffset estimation and phase offset estimation for the 5-MHz subchannel.The HE-LTFs are used for channel estimation for the 5-MHz subchannel.Since the subchannel carries four spatial streams, as many HE-LTFs(i.e., HE-LTF symbols or HE-LTF elements in a HE-LTF section) as thenumber of the spatial streams, that is, four HE-LTFs are required tosupport MU-MIMO transmission.

According to an example of the present invention, relationship between atotal number of spatial streams transmitted on one subchannel and anumber of HE-LTFs is listed in [Table 3].

TABLE 3 Total number of spatial streams transmitted on one subchannelNumber of HE-LTFs 1 1 2 2 3 4 4 4 5 6 6 6 7 8 8 8

Referring to [Table 3], if one spatial stream is transmitted on onesubchannel, at least one HE-LTF needs to be transmitted on thesubchannel. If an even number of spatial streams are transmitted on onesubchannel, at least as many HE-LTFs as the number of the spatialstreams need to be transmitted. If an odd number of spatial streamsgreater than one are transmitted on one subchannel, at least as manyHE-LTFs as a number of adding 1 to the number of the spatial streamsneed to be transmitted.

Referring to FIG. 11 again, it is assumed that the second 5-MHzsubchannel is allocated to STA3 and STA4 and one spatial streams per STAis transmitted in the DL MU-MIMO or OFDMA mode (i.e., a total of twospatial streams are transmitted on one subchannel). In this case, twoHE-LTFs need to be transmitted on the second subchannel, however, in theexample of FIG. 11, a HE-STF, a HE-LTF, a HE-LTF, a HE-LTF, a HE-LTF,and a HE-SIG-B follow the HE-SIG-A field on the subchannel (i.e., fourHE-LTFs are transmitted). This is for setting the same starting time ofPSDU transmission for subchannels allocated to other STAs paired withSTA3 and STA4 for MU-MIMO transmission. If only two HE-LTFs aretransmitted on the second subchannel, PSDUs are transmitted at differenttime points on the first and second subchannels. PSDU transmission oneach subchannel at a different time point results in discrepancy betweenOFDM symbol timings of subchannels, thereby no orthogonality ismaintained. To overcome this problem, an additional constraint need tobe imposed for HE-LTF transmission.

Basically, transmission of as many HE-LTFs as required is sufficient inan SU-MIMO or non-OFDMA mode. However, timing synchronization (oralignment) with fields transmitted on subchannels for other paired STAsis required in the MU-MIMO or OFDMA mode. Accordingly, the numbers ofHE-LTFs may be determined for all other subchannels based on asubchannel having the maximum number of streams in MU-MIMO-mode orOFDMA-mode transmission.

Specifically, the numbers of HE-LTFs may be determined for allsubchannels according to the maximum of the numbers of HE-LTFs (HE-LTFsymbols or HE-LTF elements in a HE-LTF section) required according tothe total numbers of spatial streams transmitted on each subchannel, fora set of HE STAs allocated to each subchannel. A “set of HE STAsallocated to each subchannel” is one HE STA in the SU-MIMO mode, and aset of HE STAs paired across a plurality of subchannels in the MU-MIMOmode. The ‘number of spatial streams transmitted on each subchannel’ isthe number of spatial streams transmitted to one HE STA in the SU-MIMOmode, and the number of spatial streams transmitted to a plurality of HESTAs paired on the subchannel in the MU-MIMO mode.

That is, it may be said that a HE-LTF field starts at the same timepoint and ends at the same time point in a HE PPDU for all users (i.e.HE STAs) in MU-MIMO-mode or OFDMA-mode transmission. Or it may be saidthat the lengths of HE-LTF sections are equal on a plurality ofsubchannels for all users (i.e. HE STAs) in MU-MIMO-mode or OFDMA-modetransmission. Or it may be said that the number of HE-LTF elementsincluded in each HE-LTF section is equal on a plurality of subchannelsfor all users (i.e. HE STAs) in MU-MIMO-mode or OFDMA-mode transmission.Accordingly, PSDU transmission timings may be synchronized among aplurality of subchannels for all HE STAs in MU-MIMO-mode or OFDMA-modetransmission.

As described above, the number of HE-LTF symbols (refer to FIG. 8) maybe 1, 2, 4, 6, or 8 in HE PPDU transmission in the MU-MIMO or OFDMAmode, determined according to the maximum of the numbers of spatialstreams on each of a plurality of subchannels. A different number ofspatial streams may be allocated to each of a plurality of subchannels,and the number of spatial streams allocated to one subchannel is thenumber of total spatial streams for all users allocated to thesubchannel. That is, the number of HE-LTF symbols may be determinedaccording to the number of spatial streams allocated to a subchannelhaving a maximum number of spatial streams by comparing the number oftotal spatial streams for all users allocated to one of a plurality ofsubchannels with the number of total spatial streams for all usersallocated to another subchannel.

Specifically, in HE PPDU transmission in the OFDMA mode, the number ofHE-LTF symbols may be 1, 2, 4, 6, or 8, determined based on the numberof spatial streams transmitted in a subchannel having a maximum numberof spatial streams across a plurality of subchannels. Further, in HEPPDU transmission in the OFDMA mode, the number of HE-LTF symbols may bedetermined based on whether the number of spatial streams transmitted ina subchannel having a maximum number of spatial streams across aplurality of subchannels is odd or even (refer to [Table 3]). That is,in HE PPDU transmission in the OFDMA mode, when the number (e.g., K) ofspatial streams transmitted in a subchannel having a maximum number ofspatial streams across a plurality of subchannels is an even number, thenumber of HE-LTF symbols may be equal to K. In HE PPDU transmission inthe OFDMA mode, when the number, K, of spatial streams transmitted in asubchannel having a maximum number of spatial streams across a pluralityof subchannels is an odd number greater than one, the number of HE-LTFsymbols may be equal to K+1.

When only one STA is allocated to one subchannel in OFDMA mode (i.e.,OFDMA mode without using MU-MIMO), a subchannel having a maximum numberof spatial streams across a plurality of subchannels may be determinedby the number of spatial streams for a STA allocated to each subchannel.When more than one STA is allocated to one subchannel in OFDMA mode(i.e., OFDMA mode using MU-MIMO), a subchannel having a maximum numberof spatial streams across a plurality of subchannels may be determinedby the number of STAs allocated to each subchannel and the number ofspatial streams for each STA allocated to each subchannel (e.g., if STA1and STA2 are allocated to one subchannel, sum of the number of spatialstreams for STA1 and the number of spatial streams for STA2).

When transmitting a HE PPDU frame in the MU-MIMO or OFDMA mode, atransmitter may generate P (P is an integer equal to or larger than 1)HE-LTF symbols (refer to FIG. 8) and transmit a HE PPDU frame includingat least the P HE-LTF symbols and a Data field to a receiver. The HEPPDU frame may be divided into Q subchannels in the frequency domain (Qis an integer equal to or larger than 2). Each of the P HE-LTF symbolsmay be divided into Q HE-LTF elements corresponding to the Q subchannelsin the frequency domain. That is, the HE PPDU may include P HE-LTFelements on one subchannel (herein, the P HE-LTF elements may belong toone HE-LTF section on the subchannel).

As described above, the number of HE-LTF elements (i.e., P) in one ofthe Q subchannels may be equal to the number of HE-LTF elements (i.e. P)of another subchannel. Also, the number of HE-LTF elements (i.e., P)included in a HE-LTF section in one of the Q subchannels may be equal tothe number of HE-LTF elements (i.e. P) included in a HE-LTF section inanother subchannel. The HE-LTF section of one of the Q subchannels maystart and end at the same time points as the HE-LTF section of anothersubchannel. Also, the HE-LTF sections may start and end at the same timepoints across the Q subchannels (i.e., across all users or stations).

Referring to FIG. 11 again, the third 5-MHz subchannel is allocated toSTA5 and one spatial stream is transmitted on the subchannel in SU-MIMO(considering all subchannels, a plurality of spatial streams aretransmitted to STA1 to STA6 in MU-MIMO or OFDMA mode). In this case,although transmission of one HE-LTF is sufficient for the subchannel, asmany HE-LTFs as the maximum of the numbers of HE-LTFs on the othersubchannels, that is, four HE-LTFs are transmitted on the subchannel inorder to align the starting points and ending points of the HE-LTFfields of the subchannels.

The fourth 5-MHz subchannel is allocated to STA6 and one spatial streamis transmitted on the subchannel in SU-MIMO (considering all othersubchannels, a plurality of spatial streams are transmitted to STA1 toSTA6 in MU-MIMO or OFDMA mode). In this case, although transmission ofone HE-LTF is sufficient for the subchannel, as many HE-LTFs as themaximum of the numbers of HE-LTFs on the other subchannels, that is,four HE-LTFs are transmitted on the subchannel in order to align thestarting points and ending points of the HE-LTF fields of thesubchannels.

In the example of FIG. 11, the remaining two HE-LTFs except two HE-LTFsrequired for channel estimation of STA3 and STA4 on the secondsubchannel, the remaining three HE-LTFs except one HE-LTF required forchannel estimation of STA5 on the third subchannel, and the remainingthree HE-LTFs except one HE-LTF required for channel estimation of STA6on the fourth subchannel may be said to be placeholders that areactually not used for channel estimation at the STAs.

FIG. 12 depicts a HE-SIG-B field and a HE-SIG-C field in the HE PPDUframe format according to the present invention.

To effectively support MU-MIMO-mode or OFDMA-mode transmission in the HEPPDU frame format according to the present invention, independentsignaling information may be transmitted on each subchannel.Specifically, a different number of spatial streams may be transmittedto each of a plurality of HE STAs that receive an MU-MIMO-mode orOFDMA-mode transmission simultaneously. Therefore, information about thenumber of spatial streams to be transmitted should be indicated to eachHE STA.

Information about the number of spatial streams on one channel may beincluded in, for example, a HE-SIG-A field. A HE-SIG-B field may includespatial stream allocation information about one subchannel. Also, aHE-SIG-C field may be transmitted after transmission of HE-LTFs,including Modulation and Coding Scheme (MCS) information about a PSDUand information about the length of the PSDU, etc.

With reference to the foregoing examples of the present invention,mainly the features of a HE PPDU frame structure applicable to a DLMU-MIMO-mode or OFDMA-mode transmission that an AP transmitssimultaneously to a plurality of STAs have been described. Now, adescription will be given of the features of a HE PPDU frame structureapplicable to a UL MU-MIMO-mode or OFDMA-mode transmission that aplurality of STAs transmits simultaneously to an AP.

The above-described various examples of structures of the HE PPDU frameformat supporting MU-MIMO-mode or OFDMA-mode transmission should not beunderstood as applicable only to DL without applicable UL. Rather, theexamples should be understood as also applicable to UL. For example, theabove-described exemplary HE PPDU frame formats may also be used for aUL HE PPDU transmission that a plurality of STAs simultaneouslytransmits to a single AP.

However, in the case of a DL MU-MIMO-mode or OFDMA-mode HE PPDUtransmission that an AP simultaneously transmits to a plurality of STAs,the transmission entity, AP has knowledge of the number of spatialstreams transmitted to a HE STA allocated to each of a plurality ofsubchannels. Therefore, the AP may include, in a HE-SIG-A field or aHE-SIG-B field, information about the total number of spatial streamstransmitted across a channel, a maximum number of spatial streams (i.e.,information being a basis of the number of HE-LTF elements (or thestarting point and ending point of a HE-LTF section) on eachsubchannel), and the number of spatial streams transmitted on eachsubchannel. In contrast, in the case of a UL MU-MIMO-mode or OFDMA-modeHE PPDU transmission that a plurality of STAs simultaneously transmitsto an AP, each STA being a transmission entity may be aware only of thenumber of spatial streams in a HE PSDU that it will transmit, withoutknowledge of the number of spatial streams in a HE PSDU transmitted byanother STA paired with the STA. Accordingly, the STA may determineneither the total number of spatial streams transmitted across a channelnor a maximum number of spatial streams.

To solve this problem, a common parameter (i.e., a parameter appliedcommonly to STAs) and an individual parameter (a separate parameterapplied to an individual STA) may be configured as follows in relationto a UL HE PPDU transmission.

For simultaneous UL HE PPDU transmissions from a plurality of STAs to anAP, a protocol may be designed in such a manner that the AP sets acommon parameter or individual parameters (common/individual parameters)for the STAs for the UL HE PPDU transmissions and each STA operatesaccording to the common/individual parameters. For example, the AP maytransmit a trigger frame (or polling frame) for a UL MU-MIMO-mode orOFDMA-mode transmission to a plurality of STAs. The trigger frame mayinclude a common parameter (e.g., the number of spatial streams across achannel or a maximum number of spatial streams) and individualparameters (e.g., the number of spatial streams allocated to eachsubchannel), for the UL MU-MIMO-mode or OFDMA-mode transmission. As aconsequence, a HE PPDU frame format applicable to a UL MU-MIMO or OFDMAmode may be configured without a modification to an exemplary HE PPDUframe format applied to a DL MU-MIMO or OFDMA mode. For example, eachSTA may configure a HE PPDU frame format by including information aboutthe number of spatial streams across a channel in a HE-SIG-A field,determining the number of HE-LTF elements (or the starting point andending point of a HE-LTE section) on each subchannel according to themaximum number of spatial streams, and including information about thenumber of spatial streams for the individual STA in a HE-SIG-B field.

Alternatively, if the STAs operate always according to thecommon/individual parameters received in the trigger frame from the AP,each STA does not need to indicate the common/individual parameters tothe AP during a HE PPDU transmission. Therefore, this information maynot be included in a HE PPDU. For example, each STA may have only todetermine the total number of spatial streams, the maximum number ofspatial streams, and the number of spatial streams allocated toindividual STA, as indicated by the AP, and configure a HE PPDUaccording to the determined numbers, without including information aboutthe total number of spatial streams or the number of spatial streamsallocated to the STA in the HE PPDU.

On the other hand, if the AP does not provide common/individualparameters in a trigger frame, for a UL MIMO-mode or OFDMA-mode HE PPDUtransmission, the following operation may be performed.

Common transmission parameters (e.g., channel BandWidth (BW)information, etc.) for simultaneously transmitted HE PSDUs may beincluded in HE-SIG-A field, but parameters that may be different forindividual STAs (e.g., the number of spatial streams, an MCS, andwhether STBC is used or not, for each individual STA) may not beincluded in HE-SIG-A field. Although the individual parameters may beincluded in HE-SIG-B field, information about the number of spatialstreams and information indicating whether STBC is used or not, need tobe transmitted before a HE-LTF field because the number of spatialstreams and the information indicating whether STBC is used or not aresignificant to determination of configuration information about apreamble and a PSDU in a HE PPDU frame format (e.g., the number ofHE-LTF elements is determined according to a combination of the numberof spatial streams and the information indicating whether STBC is usedor not). For this purpose, a HE PPDU frame format as illustrated in FIG.13 may be used for a UL HE PPDU transmission.

FIG. 13 depicts another exemplary HE PPDU frame format according to thepresent invention. The HE PPDU frame format illustrated in FIG. 13 ischaracterized in that a structure of HE-SIG-A, HE-SIG-B, and HE-SIG-Cfields similar to in FIG. 12 is used for a UL PPDU transmission.

As described before, if a UL MU-MIMO-mode or OFDMA-mode transmission isperformed by triggering of an AP (according to common/individualparameters provided by the AP), an individual STA may not need to reportan individual parameter to the AP. In this case, one or more of aHE-SIG-B field, a HE-SIG-C field, and a first HE-LTF element (i.e., aHE-LTF between a HE-STF field and a HE-SIG-B field) illustrated in FIG.13 may not exist. In this case, a description of each field given belowmay be applied only in the presence of the field.

In the example of FIG. 13, a HE-SIG-A field is transmitted per channel(i.e., per 20-MHz channel) and may include transmission parameterscommon to simultaneously transmitted HE PSDUs. Since the sameinformation is transmitted in up to HE-SIG-A fields in UL PPDUstransmitted by HE STAs allocated to subchannels, the AP may receive thesame signals from the plurality of STAs successfully.

A HE-SIG-B field is transmitted per subchannel in one channel. TheHE-SIG-B field may have an independent parameter value according to thetransmission characteristics of a HE PSDU transmitted on eachsubchannel. The HE-SIG-B field may include spatial stream allocationinformation and information indicating whether STBC is used or not, foreach subchannel. If MU-MIMO is applied to a subchannel (i.e., if aplurality of STAs perform transmission on a subchannel), the HE-SIG-Bfield may include a common parameter for the plurality of STAs paired onthe subchannel.

A HE-SIG-C field is transmitted on the same subchannel as the HE-SIG-Bfield and may include information about an MCS and a packet length. IfMU-MIMO is applied to a subchannel (i.e., if a plurality of STAs performtransmission on a subchannel), the HE-SIG-C field may include respectiveindividual parameters for each of the plurality of STAs paired on thesubchannel.

Similarly to DL MU-MIMO-mode or OFDMA-mode HE PPDU transmission,transmissions of PSDUs may start at different time points on subchannelsin UL MU-MIMO-mode or OFDMA-mode HE PPDU transmission, and if OFDMsymbols are not aligned accordingly, then the implementation complexityof an AP that receives a plurality of PSDUs increased. To solve thisproblem, ‘the number of HE-LTFs may be determined for all subchannelsaccording to the maximum of the numbers of HE LTFs required according tothe total numbers of spatial streams transmitted on each subchannel fora set of HE STAs allocated to each of subchannels’ as described withreference to the example of FIG. 11.

This feature may mean that the HE-LTF field start at the same time pointand end at the same time point across all users (i.e., HE STAs) in ULMU-MIMO-mode or OFDMA-mode transmission. Or it may be said that theHE-LTF sections of a plurality of subchannels have the same lengthacross all HE STAs in UL MU-MIMO-mode or OFDMA-mode transmission. Or itmay be said that each of the HE-LTF sections of a plurality ofsubchannels includes the same number of HE-LTF elements across all HESTAs in UL MU-MIMO-mode or OFDMA-mode transmission. Therefore, PSDUtransmission timings are synchronized between a plurality of subchannelsacross all HE STAs in UL MU-MIMO-mode or OFDMA-mode transmission.

As described before, a plurality of STAs may simultaneously transmitPSDUs in a HE PPDU frame format on their allocated subchannels or ontheir allocated spatial streams to an AP (i.e., referred to as ULMU-MIMO or OFDMA transmission or “UL MU transmission”) and maysimultaneously receive PSDUs in the HE PPDU frame format on theirallocated subchannels on their allocated spatial streams from the AP(i.e., referred to as DL MU-MIMO or OFDMA transmission or “DL MUtransmission”).

FIGS. 14 and 15 depict operating channels in a WLAN system.

Basically, the WLAN system may support a single channel having abandwidth of 20 MHz as a BSS operating channel. The WLAN system may alsosupport a BSS operating channel having a bandwidth of 40 MHz, 80 MHz, or160 MHz by bonding a plurality of contiguous 20-MHz channels (refer toFIG. 14). Further, the WLAN system may support a BSS operating channelhaving a bandwidth of 160 MHz including non-contiguous 80-MHz channels(called a bandwidth of 80+80 MHz) (refer to FIG. 15).

As illustrated in FIG. 14, one 40-MHz channel may include a primary20-MHz channel and a secondary 20-MHz channel which are contiguous. One80-MHz channel may include a primary 40-MHz channel and a secondary40-MHz channel which are contiguous. One 160-MHz channel may include aprimary 80-MHz channel and a secondary 80-MHz channel which arecontiguous. As illustrated in FIG. 15, one 80+80-MHz channel may includea primary 80-MHz channel and a secondary 80-MHz channel which arenon-contiguous.

A primary channel is defined as a common channel for all STAs within aBSS. The primary channel may be used for transmission of a basic signalsuch as a beacon. The primary channel may also be a basic channel usedfor transmission of a data unit (e.g., a PPDU). If an STA uses a channelwidth larger than the channel width of the primary channel, for datatransmission, the STA may use another channel within a correspondingchannel, in addition to the primary channel. This additional channel isreferred to as a secondary channel.

A STA according to an Enhanced Distributed Channel Access (EDCA) schememay determine a transmission bandwidth (or a transmission channel width)as follows.

Upon generation of a transmission frame, an STA (e.g., an AP or a non-APSTA) may perform a back-off procedure on a primary channel in order toacquire a Transmission Opportunity (TXOP). For this purpose, the STA maysense the primary channel during a DIFS or AIFS[i]. If the primarychannel is idle, the STA may attempt to transmit the frame. The STA mayselect a random back-off count, wait for a slot time corresponding tothe selected random back-off count, and then attempt to transmit theframe. The random back-off count may be determined to be a value rangingfrom 0 to CW (CW is a value of a contention window parameter).

When the random back-off procedure starts, the STA may activate aback-off timer according to the determined back-off count and decrementthe back-off count by 1 each time. If the medium of the correspondingchannel is monitored as busy, the STA discontinues the count-down andwaits. If the medium is idle, the STA resumes the count-down. If theback-off timer reaches 0, the STA may determine a transmission bandwidthby checking whether the secondary channel is idle or busy at thecorresponding time point.

For example, the STA may monitor a channel-idle state during apredetermined IFS (e.g., DIFS or AIFS[i]) on the primary channel anddetermine a transmission start timing on the primary channel by therandom back-off procedure. If the secondary channel is idle during aPIFS shortly before the determined transmission start timing of theprimary channel, the STA may transmit a frame on the primary channel andthe secondary channel.

As described above, when the back-off timer reaches 0 for the primarychannel, the STA may transmit an X-MHz mask PPDU (e.g., X is 20, 40, 80,or 160) on channels including an idle secondary channel(s) according tothe CCA result of the secondary channel(s).

The X-MHz mask PPDU is a PPDU for which a TXVECTOR parameter, CHBANDWIDTH is set to CBW X. That is, if the X-MHz mask PPDU can betransmitted, this means that a PPDU satisfying a spectrum mask for X-MHztransmission can be transmitted. The X-MHz mask PPDU may include a PPDUtransmitted in a bandwidth equal to or smaller than X MHz.

For example, if an 80-MHz mask PPDU can be transmitted, this means thata PPDU having a channel width of 80 MHz or a PPDU having a channel widthsmaller than 80 MHz (e.g., 40 MHz, 20 MHz, etc.) can be transmittedwithin a Power Spectral Density (PSD) limit of a spectrum mask for80-MHz transmission.

As described before, if a STA is allowed to start a TXOP and has atleast one MAC Service Data Unit (MSDU) to be transmitted under theAccess Category (AC) of the TXOP allowed for the STA, the STA mayperform one of the following a), b), c), d), or e) (in the followingdescription, FIGS. 14 and 15 may be referred to for a primary channel(i.e., a primary 20-MHz channel) a secondary channel (i.e., a secondary20-MHz channel), a secondary 40-MHz channel, and a secondary 80-MHzchannel).

a) If the secondary channel, the secondary 40-MHz channel, and thesecondary 80-MHz channel are idle during a PIFS shortly before the startof the TXOP, a 160-MHz or 80+80-MHz mask PPDU may be transmitted.

b) If both the secondary channel and the secondary 40-MHz channel areidle during the PIFS shortly before the start of the TXOP, an 80-MHzmask PPDU may be transmitted on a primary 80-MHz channel.

c) If the secondary channel is idle during the PIFS shortly before thestart of the TXOP, a 40-MHz mask PPDU may be transmitted on a primary40-MHz channel.

d) A 20-MHz mask PPDU may be transmitted on the primary 20-MHz channel.

e) A channel access attempt may be resumed by performing a back-offprocedure as in the case where the medium is indicated as busy on theprimary channel by one of physical carrier sensing and virtual carriersensing and a back-off timer has a value of 0.

Now, a description will be given of a method for performing UL SU PPDUtransmission or UL MU PPDU transmission according to the type of aresponse to DL MU transmission and a method for protecting a DL/UL HEPPDU in a WLAN system supporting DL/UL MU transmission.

The method for protecting a DL/UL HE PPDU will first be described below.

Transmission of a HE PPDU may be protected by preventing another STA(e.g., a third-party STA) from accessing a wireless channel duringtransmission of the HE PPDU. For this purpose, a specific field includedin the HE PPDU may be used.

That is, the third-party STA may regard the channel as busy during acorresponding time period based on duration information included in aPHY header (e.g., an L-SIG, a HE-SIG-A, a HE-SIG-B, etc.) of the HEPPDU.

For example, the third-party STA (e.g., including a legacy STA that isnot capable of decoding a HE preamble and a data part of the HE PPDU anda HE STA capable of decoding the HE PPDU) may not perform transmissionduring the corresponding time period based on a duration (i.e., an L-SIGduration) determined according to a parameter (e.g., a L_LENGTH subfieldor an L_DATARATE subfield) included in the L-SIG field of the HE PPDU.As a consequence, since the third-party STA does not attempt to accessthe channel during the time period determined based on the L-SIGduration even though the channel is physically idle, the HE PPDUtransmission may be protected.

For example, the parameter, L_LENGTH or L_DATARATE included in the L-SIGfield may be set as follows by [Equation 1].

$\begin{matrix}{{L\_ LENGTH} = {{\left\lceil \frac{\begin{pmatrix}{\left( {{TXTIME} - {{Signal}\mspace{14mu} {Extension}}} \right) -} \\\left( {{aPreambleLength} + {aPHYHeaderLength}} \right)\end{pmatrix}}{aSymbolLength} \right\rceil \times N_{OPS}} - \left\lceil \frac{\begin{matrix}{{aPHYServiceLength} +} \\{aPHYConvolutionTailLength}\end{matrix}}{8} \right\rceil}} & \left\lbrack {{Equation}\mspace{14mu} 1} \right\rbrack\end{matrix}$

In [Equation 1], ┌ ┐ represents a ceiling operation and 1×1 representsthe least integer equal to or larger than x.

If a TXVECTOR parameter, NO_SIG_EXTN is set to True, SignalExtension mayhave a value of 0 μs. On the other hand, if the TXVECTOR parameter,NO_SIG_EXTN is set to False, SignalExtension may have a valuecorresponding to a duration defined by an aSignalExtension parameter (6μs in 2.4 GHz and 0 μs in 5 GHZ).

aSymbolLength may have a value corresponding to a symbol duration (inμs). In general, aSymbolLength may have a fixed value of 4.

(aPreambleLength+aPHYHeaderLength) may have a value corresponding to thedurations of a non-HT PHY preamble and an L-SIG (in μs), and may followwhat is defined in a PLME-CHARACTERISTICS.confirm primitive.

If a rate specified by L_DATARATE is used, N_(OPS) may have a valuecorresponding to the number of octets transmitted during a time periodindicated by aSymbolLength. In general, L_DATARATE may have a fixedvalue of 6 Mbps. In this case, N_(OPS) may have a fixed value of 4.

aPHYServiceLength may have a value corresponding to the number of bitsof a PHY SERVICE field.

aPHYConvolutionalTailLength may have a value corresponding to the numberof bits of a convolutional code tail bit sequence.

In the case of a HE PPDU, if (aPreambleLength+aPHYHeaderLength)=20,aSymbolLength=4, N_(OPS)=3, and24≦(aPHYServiceLength+aPHYConvolutionalTailLength)<32 in the above[Equation 1], the value of the L_LENGTH subfield may be expressed as[Equation 2].

$\begin{matrix}{{L\_ LENGTH} = {{\left\lceil \frac{\left( {\left( {{TXTIME} - {{Signal}\mspace{14mu} {Extension}}} \right) - 20} \right)}{4} \right\rceil \times 3} - 3}} & \left\lbrack {{Equation}\mspace{14mu} 2} \right\rbrack\end{matrix}$

In [Equation 2], TXTIME represents the duration of the HE PPDU, definedby [Equation 3].

${TXTIME} = {T_{L\text{-}{STF}} + T_{L\text{-}{LTF}} + T_{L\text{-}{SIG}} + T_{{HE}\text{-}{SIG}\text{-}A} + \left\lceil \frac{T_{{HE}\text{-}{STF}} + {N_{HELTF} \times T_{{HE}\text{-}{LTF}}} + T_{{HE}\text{-}{SIG}\text{-}B} + {N_{SYM} \times T_{STMD}}}{T_{SYML}} \right\rceil}$

In [Equation 3], T_(L-STF) may have a value corresponding to theduration of a Non-HT STF field (e.g., 8 μs).

T_(L-LTF) may have a value corresponding to the duration of a Non-HT LTFfield (e.g., 8 μs).

T_(L-SIG) may have a value corresponding to the duration of a Non-HTSIGNAL field (e.g., 4 μs).

T_(HE-SIG-A) may have a value corresponding to the duration of aHE-SIG-A field (e.g., 8 μs or 12 μs). Or T_(HE-SIG-A) may have a valuebased on a duration of N_(HESIGA)*4 μs and a GI of 0.8 μs, asillustrated in [Table 1].

If a HE-SIG-B field is not included in the HE PPDU, T_(HE-SIG-B) mayhave a value of 0, and if the HE-SIG-B field is included in the HE PPDU,T_(HE-SIG-B) may have a value corresponding to the duration of theHE-SIG-B field (e.g., 16 μs or 15.6 μs). Or T_(HE-SIG-B) may have avalue based on a duration of N_(HESIGB)*4 μs and a GI of 0.8 μs, asillustrated in [Table 1].

T_(HE-STF) may have a value corresponding to the duration of a HE-STFfield (e.g., 16 μs or 15.6 μs). Or T_(HE-STF) may have a value of 4 μsor 8 μs, as illustrated in [Table 1].

T_(HEW-LTF) may have a value corresponding to the duration of a HE-LTFfield (e.g., 16 μs or 15.6 μs). Or T_(HEW-LTF) may have a value ofN_(HELTF)*(DFT interval+GI)μ, as illustrated in [Table 1].

N_(HELTF) may have a value corresponding to the number of HE-LTFsymbols.

T_(SYMD) may have a value corresponding to a double GI symbol interval(e.g., 16 μs or 15.6 μs).

T_(SYML) may have a value corresponding to a long GI symbol interval(e.g., 4 μs).

N_(SYM) may have a value corresponding to the number of symbols of aDATA (or HE-DATA) field.

As illustrated in [Equation 3], since the HE-STF, HE-LTF, HE-SIG-B, andDATA (or HE-DATA) fields have variable durations (e.g. in view of anincluded variable GI) in the HE PPDU, a ceiling operation may be appliedin consideration that division of the durations by the number of OFDMsymbols results in a remainder.

Further, T_(RL-SIG) having a value corresponding to the duration of anRL-SIG field of the HE PPDU may be added to TXTIME depicted in [Equation3].

In another example of protecting transmission of a HE PPDU using aspecific field included in the HE PPDU from a third-party STA, aDuration/ID field (or a Duration field) included in the MAC header of adata unit (e.g., a PSDU field, a DATA field, or a HE-DATA field) in theHE PPDU may be used. As a third-party STA (e.g., a HE STA capable ofdecoding the HE PPDU) sets a NAV, the Duration field of the MAC headermay induce the third-party STA to regard a channel as busy for acorresponding time period. Therefore, even though the channel isphysically idle during a time period to which the NAV is set, thethird-party STA may not attempt to access the channel, therebyprotecting the HE PPDU transmission.

FIG. 16 depicts a NAV update operation of an STA according to thepresent invention.

Upon detection of a PPDU (S1610), the STA may determine the type of thePPDU (S1620). According to an embodiment, the PPDU may have a first typeor a second type. Specifically, the first type may be a non-OFDMA PPDUtype or an SU type, and the second type may be an OFDMA PPDU type or anMU type.

If the types allowed for a PPDU include the non-OFDMA PPDU type as thefirst type and the OFDMA PPDU type as the second type, the STA mayperform NAV update depending on whether the received PPDU is an OFDMAPPDU. The STA may determine whether the received PPDU is an OFDMA PPDUby checking whether a transmission channel bandwidth of the (HE) PPDU isequal to a transmission channel bandwidth of a PSDU. A NAV may beupdated in different manners in the cases of reception of an OFDMA PPDUand reception of a non-OFDMA PPDU.

If the PPDU is of the first type, particularly the non-OFDMA PPDU type,the STA performs a NAV update operation for a first-type PPDU (non-OFDMAPPDU) (S1630). When the PPDU is of the first type, the STA performs NAVupdate based on the Duration/ID field of a frame received on a primarychannel. If the transmission channel bandwidth of a (HE) PPDU is equalto the transmission channel bandwidth of a PSDU, a (HE) STA receivingthe (HE) PPDU may set a NAV as defined as follows: If a Receiver Address(RA) matching to the MAC address of the (HE) STA is not included in anyframe received in a 20-MHz (HE) PPDU on a primary 20-MHz channel, in a40-MHz (HE) PPDU on a primary 40-MHz channel, in a 80-MHz (HE) PPDU on aprimary 80-MHz channel, or in a 160-MHz or 80+80-MHz (HE) PPDU, a NAV ofthe (HE) STA is updated using the Duration/ID field (or the Durationfield) of the frame. Specifically, if the STA receives a frame thatbelongs to a secondary channel (a secondary 20-MHz channel, a secondary40-MHz channel, or a secondary 80-MHz channel) but does not belong to aprimary channel (a primary 20-MHz channel, a primary 40-MHz channel, ora primary 80-MHz channel), the STA does not perform the NAV update.

On the other hand, if the PPDU is of the second type, particularly theOFDMA PPDU type, the STA performs a NAV update operation for asecond-type PPDU (OFDMA PPDU) (S1640). If the PPDU is of the secondtype, the STA may perform NAV update based on the Duration/ID field of aframe received on a subchannel, irrespective of a channel to which thereceived subchannel belongs (i.e., without considering whether thereceived subchannel carrying the PPDU belongs to a primary channel or asecondary channel). In the case of a HE PPDU supporting DL/UL OFDMAtransmission, the channel bandwidth of a PSDU for a specific HE STA maybe smaller than the channel bandwidth of the HE PPDU. In the example ofFIG. 13, a HE PPDU has a transmission channel bandwidth of 20 MHz, and aPSDU for STA1 has a transmission channel bandwidth (i.e., a subchannelbandwidth) of 5 MHz. If a PSDU has a smaller transmission channelbandwidth than a HE PPDU as in the example, a HE STA receiving the HEPPDU may set a NAV as defined as follows. If an RA matching to the MACaddress of the HE STA is not included in any frame received on anysubchannel of a 20-MHz HE PPDU (or a 20-MHz OFDMA PPDU) on a primary20-MHz channel, any subchannel of a 40-MHz HE PPDU (or a 40-MHz OFDMAPPDU) on a primary 40-MHz channel, any subchannel of a 80-MHz HE PPDU(or a 80-MHz OFDMA PPDU) on a primary 80-MHz channel, or any subchannelof a 160-MHz or 80+80-MHz HE PPDU (or a 160-MHz or 80+80-MHz OFDMAPPDU), the HE STA updates its NAV using the Duration/ID field (or theDuration field) of the frame.

Specifically, if a HE STA receives a frame including an RA that does notmatch to the MAC address of the HE STA on any subchannel, the HE STAupdates its NAV using the Duration/ID field (or the Duration field) ofthe frame, irrespective of whether the subchannel belongs to the primary20-MHz channel, the secondary 20-MHz channel, the secondary 40-MHzchannel, or the secondary 80-MHz channel.

More specifically, if the RA matching to the MAC address of the HE STAis not included in any frame received on any subchannel of a 40-MHz HEPPDU (or a 40-MHz OFDMA PPDU) on the primary 40-MHz channel, the HE STAupdates its NAV using the Duration/ID field (or the Duration field) ofthe frame, irrespective of whether the subchannel belongs to the primary20-MHz channel or the secondary 20-MHz channel. If the RA matching tothe MAC address of the HE STA is not included in any frame received onany subchannel of a 80-MHz HE PPDU (or a 80-MHz OFDMA PPDU) on theprimary 80-MHz channel, the HE STA updates its NAV using the Duration/IDfield (or the Duration field) of the frame, irrespective of whether thesubchannel belongs to the primary 20-MHz channel, the secondary 20-MHzchannel, or the secondary 40-MHz channel. If the RA matching to the MACaddress of the HE STA is not included in any frame received on anysubchannel of a 160-MHz HE PPDU (or a 160-MHz OFDMA PPDU) or a 80+80-MHzHE PPDU (or a 80+80-MHz OFDMA PPDU) on a 160-MHz or 80+80-MHz channel,the HE STA updates its NAV using the Duration/ID field (or the Durationfield) of the frame, irrespective of whether the subchannel belongs tothe primary 20-MHz channel, the secondary 20-MHz channel, the secondary40-MHz channel, or the secondary 80-MHz channel.

The HE PPDU may also include a resource unit having a channel bandwidthequal to or smaller than the channel bandwidth of the HE PPDU. HE PPDUprotection may be implemented in the case where a HE PPDU is received ona secondary channel as well as in the case where a HE PPDU is receivedon a primary channel. Accordingly, NAV setting of a HE STA receiving aHE PPDU may be defined as follows. If an RA matching to the MAC addressof the HE STA is not included in any frame received in any 20-MHz orless resource unit of a 20-MHz HE PPDU (or 20-MHz OFDMA PPDU) on theprimary or secondary 20-MHz channel, in any 40-MHz or less resource unitof a 40-MHz HE PPDU (or 40-MHz OFDMA PPDU) on the primary or secondary40-MHz channel, in any 80-MHz or less resource unit of a 80-MHz HE PPDU(or 80-MHz OFDMA PPDU) on the primary or secondary 80-MHz channel, inany 160-MHz or less resource unit of a 160-MHz or 80+80-MHz HE PPDU (or160-MHz or 80+80-MHz OFDMA PPDU), the NAV of the HE STA is updated usingthe Duration/ID field (or the Duration field) of the frame.

FIG. 17 depicts an operation of a third-party STA, when a DL OFDMA PPDUis transmitted according to the present invention.

While FIG. 17 illustrates an exemplary case where the transmission timesof PSDUs are different on subchannels (i.e., the lengths of HE-LTFsections are different on the subchannels) in a DL HE PPDU formattransmitted on each of a plurality of channels, a HE PPDU format inwhich the transmission times of PSDUs are identical on subchannels(i.e., the lengths of HE-LTF sections are equal on the subchannels) asillustrated in the examples of FIGS. 11, 12, and 13, and a UL HE PPDUformat are also applicable.

FIG. 17 illustrates an operation of a third-party HE STA, STA21, when aDL OFDMA PPDU is transmitted to STA1, STA2, STA3, STA4, STA5, STA6,STA7, STA8, STA9, STA10, STA11, and STA12.

A HE preamble of the HE PPDU may include resource allocationinformation. For example, a plurality of destination STAs may acquireinformation about subchannels allocated to the STAs from the HE preamble(e.g., a HE-SIG (HE-SIG-A or HE-SIG-B) field) of the DL OFDMA PPDU. Thatis, the HE preamble of the HE PPDU may include STA identificationinformation identifying an STA(s) allocated to a specific subchannel (orresource unit).

Because the size (e.g., the number of available bits) of informationthat can be included in the HE preamble is limited, STA identificationinformation for a specific subchannel (or resource unit) may not specifyonly one STA. For example, the Association ID (AID) of an STA may bedefined in 16 bits. If STA identification information identifying an STAallocated to a specific resource unit includes a partial AID (e.g., X(X<16) Least Significant Bits (LSBs) of an AID), one piece of STAidentification information may identify a plurality of STAs. Or if anSTA belonging to a BSS (OBSS) overlapping with a BSS of an AP receives aDL OFDMA PPDU, STAs of the different BSSs may correspond to one piece ofSTA identification information.

In this case, although an STA receiving the DL OFDMA PPDU may determinethat it is a destination STA of the DL OFMDA PPDU based on the resourceallocation information (e.g., STA identification information for aspecific subchannel (or resource unit)) included in the HE preamble, theSTA may not be an STA to which the AP has actually allocated resources.That is, although resources are not actually allocated to the STA, theSTA may consider that resources are allocated to it based on theresource allocation information of the DL OFDMA PPDU.

The example of FIG. 17 illustrates a case in which subchannel allocationinformation is shared between STA11 and STA21. That is, although STA21may consider that a subchannel is allocated to STA21 based oninformation of a HE preamble, STA11 and STA12 may not actually beserviced simultaneously in a DL OFDMA PPDU.

Specifically, although resources of a third 5-MHz subchannel of a second20-MHz channel (an upper 20-MHz channel in FIG. 17) are actuallyallocated to STA11, STA21 may determine that the subchannel resourcesare allocated to STA21 from the HE preamble (e.g., HE-SIG-A) of the DLOFDMA PPDU in the example of FIG. 17. Therefore, after receiving theHE-SIG-A field, STA21 may move to its allocated subchannel and start toreceive a HE-STF, a HE-LTF, a HE-SIG-B, and a PSDU. However, STA21 maybe aware from the RA field of the MAC header of the PSDU that the actualdestination STA of the PSDU is STA11. In this case, STA21 may performthe afore-described NAV update operation. That is, since the value ofthe RA field of a frame received on any subchannel of a 40-MHz OFDMAPPDU on a primary 40-MHz channel does not match to the MAC address ofSTA21, STA21 may set a NAV value based on the value of the Durationfield of the MAC header of the PSDU.

As described above, if a third-party STA receiving a HE PPDU supportingDL/UL OFDMA determines that the third-party STA is not an actualdestination STA of a frame received even on a part (e.g., a subchannel)of a transmission channel bandwidth of the HE PPDU (e.g., the RA valueof the received frame does not match to the address of the third-partySTA), the third-party STA may perform NAV update based on the Durationfield of the MAC header of the frame.

Even though the HE PPDU supporting DL/UL OFDMA received at thethird-party STA has been transmitted on a secondary 20-MHz, 40-MHz, or80-MHz channel, not on a primary 20-MHz, 40-MHz, or 80-MHz channel, ifthe third-party STA determines that the third-party STA is not an actualdestination of the received frame (e.g., the RA value of the receivedframe does not match to the address of the third-party STA), thethird-party STA may perform NAV update based on the Duration field ofthe MAC header of the frame.

In the example of FIG. 17, if resource allocation information for theother third-party STA(s) except for STA21 is not included in thereceived PPDU, the other third-party STA(s) may determine that the otherthird-party STA(s) is not a destination STA of the HE PPDU. In thiscase, the other STA(s) may not perform transmission during a time perioddetermined based on duration information included in a PHY header (e.g.,L-SIG, HE-SIG-A, HE-SIG-B) and may not process a data unit (e.g. a PSDU)following the PHY header. Meanwhile, although resource allocationinformation for STA21 is included in the received PPDU and thus STA21determines that it is a destination STA of the HE PPDU, STA21 mayfinally determine that it is not a destination STA by checking the RAfield of the MAC header of the received frame and thus may perform NAVupdate using the Duration field of the MAC header in the example of FIG.17.

FIG. 18 depicts a NAV update operation of an STA according to thepresent invention.

Upon detection of reception of a HE PPDU in step S1810, an STA maydetermine whether the detected HE PPDU is of the first type (e.g., thenon-OFDMA PPDU type) or of the second type (e.g., the OFDMA PPDU type)in step S1820.

If the type of the PPDU is the first type, particularly the non-OFDMAPPDU type, the STA may determine whether the value of the RA field ofthe MAC header of a data unit in the non-OFDMA PPDU matches to anaddress of the STA in step S1830.

If the STA determines that the value of the RA field of the MAC headerof the data unit matches to the address of the STA, the STA may process(e.g., decode) the data unit in step S1840.

On the contrary, if the STA determines that the value of the RA field ofthe MAC header of the data unit does not match to the address of theSTA, the STA may perform NAV update according to a NAV update operationfor an STA receiving a non-OFDMA PPDU in step S1850. Specifically, theSTA performs NAV update based on the Duration/ID field of a framereceived on a primary channel. If the STA receives a frame that belongsto a secondary channel, not the primary channel, the STA does notperform NAV update. The examples described in relation to step S1630 ofFIG. 16 are also applicable to step S1850 of FIG. 18.

If the type of the PPDU is the second type, particularly the OFDMA PPDUtype, the STA may determine whether it is a destination STA of the PPDUin step S1860. The STA may determine whether it is a destination STA ofthe PPDU based on information included in the PHY header (e.g., HE-SIG-Aor HE-SIG-B) of the detected PPDU. For example, upon detection of the HEPPDU, if resource allocation information included in the HE-SIG-A orHE-SIG-B field of the HE PPDU indicates the presence of a resource unitallocated to the STA, the STA may determine that it is a destination STAof the HE PPDU, and otherwise, the STA may determine that it is not adestination STA of the HE PPDU.

If the STA determines that it is not a destination STA of the HE PPDU instep S1860, the STA may not process a data unit (e.g., a PSDU) followingthe PHY header in step S1865. In addition, the STA may not performtransmission during a time period determined based on durationinformation included in the PHY header (e.g., L-SIG, HE-SIG-A, orHE-SIG-B).

If the STA determines that it is a destination STA of the HE PPDU instep S1860, the STA may determine whether the value of the RA field ofthe MAC header of a data unit in the OFDMA PPDU (i.e., a data unitreceived in a resource unit indicated by the resource allocationinformation of the HE preamble of the OFDMA PPDU) matches to the addressof the STA in step S1870.

If the value of the RA field of the MAC header of the data unit receivedin the resource unit allocated to the STA matches to the address of theSTA, the STA may process (e.g., decode) the data unit received in theresource unit allocated to the STA in step S1880.

On the contrary, if the value of the RA field of the MAC header of thedata unit received in the resource unit allocated to the STA does notmatch to the address of the STA, the STA may perform NAV updateaccording to a NAV update operation for an STA receiving an OFDMA PPDUin step S1890. That is, if the type of the PPDU is the second type, theSTA may perform NAV update based on the Duration/ID field of a framereceived on a subchannel irrespective of a channel to which the receivedsubchannel belongs (i.e., without considering whether the subchannelcarrying the PPDU belongs to a primary channel or a secondary channel).The examples described in relation to step S1640 of FIG. 16 is alsoapplicable to step S1890 of FIG. 18.

Now, a description will be given of a method for performing UL SU PPDUtransmission or UL MU PPDU transmission according to the type of aresponse to DL MU transmission. For example, while UL SU transmission ofa response to DL MU transmission is basically supported, if responses toDL MU transmission can be transmitted in UL MU transmission, systemperformance such as DL throughput may be significantly increased.

UL MU-MIMO transmission is taken as an example of UL MU transmission inthe following examples of the present invention. However, the examplesof the present invention are also applicable in the same manner to ULOFDMA transmission in which one transmission channel is divided into aplurality of subchannels and each STA performs simultaneous ULtransmission on an allocated subchannel. Similarly, DL MU-MIMOtransmission is taken as an example of DL MU transmission in thefollowing examples of the present invention. However, the examples ofthe present invention are also applicable in the same manner to DL OFDMAtransmission in which one transmission channel is divided into aplurality of subchannels and simultaneous DL transmission is performedon respective subchannels allocated to STAs. That is, UL MU transmissionincludes UL MU-MIMO transmission or UL OFDMA transmission, and DL MUtransmission includes DL MU-MIMO transmission or DL OFDMA transmission,in the following description.

FIG. 19 depicts an exemplary UL SU transmission-based ACK procedure inresponse to DL MU transmission.

In a DL MU transmission operation, an AP may transmit a DL MU PPDU todestination STAs of the DL MU transmission after exchanging an RTS frameand a CTS frame with one of the STAs of the DL MU transmission. The QoSControl fields of the MAC headers of a plurality of data units (e.g.,PSDUs) in the DL MU PPDU may include ACK Policy subfields. While the ACKPolicies of the destination STAs of the DL MU PPDU may be set to BlockACK, the ACK Policy of one of the destination STAs may be set toImplicit Block ACK Request. Thus, the STA for which the ACK Policy isset to Implicit Block ACK Request may transmit a block ACK frame to theAP a predetermined IFS (e.g., an SIFS) after receiving the DL MU PPDU,without receiving a block ACK request frame. On the other hand, anSTA(s) for which the ACK Policy is set to Block ACK may transmit a blockACK frame to the AP a predetermined IFS (e.g., an SIFS) after receivinga block ACK request frame from the AP.

The DL MU PPDU may include information requesting one STA to transmit animmediate response to the DL MU PPDU (e.g., a UL response transmitted apredetermined IFS (e.g., an SIFS) after receiving the DL MU PPDU), thatis, information triggering UL SU transmission (or a UL SU transmissiontrigger frame).

If the ACK Policy is set to Implicit Block ACK Request for two or moreof the destination STAs of the DL MU transmission, the plurality of STAsmay transmit block ACK frames simultaneously (i.e., a predetermined IFS(e.g., an SIFS) after reception of the DL MU PPDU), thereby causingcollision between them. Therefore, the ACK Policy for the DL MU PPDUshould be set to Implicit Block ACK Request only for one STA, andotherwise, Implicit Block ACK Request may not be used.

In the example of FIG. 19, before transmitting a DL MU PPDU to aplurality of STAs (e.g., STA1, STA2, STA3, and STA4) on a 40-MHzchannel, the AP may transmit an RTS frame to one (e.g., STA1) of theplurality of STAs in duplicated PPDUs on a primary 20-MHz channel (i.e.,a lower-frequency 20-MHz channel) and a secondary 20-MHz channel (i.e.,a higher-frequency 20-MHz channel). STA1 may transmit a CTS frame to theAP in duplicated PPDUs on the primary 20-MHz channel and the secondary20-MHz channel in response to the received RTS frame.

The DL MU DATA PPDU transmitted by the AP may include PSDUs directed toSTA1, STA2, STA3, and STA4. The ACK Policy of a PSDU may be set toImplicit Block ACK Request for STA1, and to Block ACK for STA2, STA3,and STA4. Therefore, STA1 may determine from the PSDU of the DL MU DATAPPDU that the ACK Policy is Implicit Block ACK Request and transmit ablock ACK PPDU to the AP a predetermined IFS (e.g., an SIFS) afterreceiving the DL MU DATA PPDU. To receive block ACK PPDUs from STA2,STA3, and STA4, the AP may transmit block ACK request PPDUs sequentiallySTA2, STA3, and STA4.

FIG. 20 depicts an exemplary UL MU transmission-based ACK procedure inresponse to DL MU transmission.

To improve the performance of a procedure for performing DL MUtransmission and transmitting an ACK in response to the DL MUtransmission, block ACKs may be transmitted in UL MU transmission. Forexample, if all of the destination STAs of a DL MU DATA PPDU support ULMU transmission, a procedure for receiving block ACK PPDUs from the STAsmay be simplified and thus the use efficiency of a wireless channel maybe increased.

As illustrated in the example of FIG. 20, after the AP and STA1 exchangean RTS frame and a CTS frame on a 40-MHz channel, the AP may transmit aDL MU DATA PPDU to STA1, STA2, STA3, and STA4 on the 40-MHz channel. Ifall of STA1, STA2, STA3, and STA4 support UL MU transmission, STA1,STA2, STA3, and STA4 may simultaneously transmit a UL MU block ACK PPDUto the AP a predetermined IFS (e.g., an SIFS) after receiving the DL MUDATA PPDU.

For example, STA1, STA2, STA3, and STA4 may be allocated distinguishablestreams and simultaneously transmit block ACK frames in a UL PPDU havinga transmission channel bandwidth of 40 MHz to the AP in UL MU-MIMOtransmission. Or STA1, STA2, STA3, and STA4 may all be allocatedsubchannels each having a bandwidth less than 40 MHz in a 40-MHz UL PPDUand transmit block ACK frames simultaneously to the AP in UL OFDMA.

To enable STA2, STA3, and STA4 to simultaneously transmit a UL MU blockACK PPDU, the ACK Policy may be set to Implicit Block ACK Request forSTA1, STA2, STA3, and STA4 in the DL MU DATA PPDU.

As described above, information eliciting a plurality of STAs totransmit immediate responses (e.g., UL responses transmitted apredetermined IFS (e.g., an SIFS) after reception of a DL MU PPDU) tothe DL MU PPDU, that is, information triggering UL MU transmission (or aUL MU transmission trigger frame) may be included in the DL MU PPDU.

As described with reference to the examples of FIGS. 19 and 20, animmediate response to a DL MU PPDU may be of a UL SU transmission typeor a UL MU transmission type. The type of the immediate response may beindicated by information included in the DL MU PPDU (e.g., a block ACKrequest). That is, information indicating the type of the immediateresponse to the DL MU PPDU (e.g., a response frame transmitted in UL SUtransmission or a response frame transmitted in UL MU transmission) maybe included in the DL MU PPDU (e.g., a block ACK request included in theDL MU PPDU). Also, the DL MU PPDU may include a block ACK request PPDUfor the plurality of STAs.

FIGS. 21 and 22 depict various types of UL responses to DL MUtransmission.

All destination STAs of a DL MU DATA PPDU may not be assumed to supportUL MU transmission. Therefore, the destination STAs of the DL MU DATAPPDU may be classified into UL MU transmission supported STAs and UL MUtransmission non-supported STAs, and a UL transmission scheme may bedetermined for the DL MU DATA PPDU accordingly.

The AP may request a UL MU transmission supported STA to transmit a ULMU block ACK PPDU (i.e., the AP may provide a response request (ortrigger) indicating the UL MU transmission-based response type).Meanwhile, the AP may request a UL MU transmission non-supported STA totransmit a UL SU block ACK PPDU (e.g., a legacy block ACK PPDU) (i.e.,the AP may provide a response request (or trigger) indicating the UL SUtransmission-based response type).

Such different types of UL responses may not be transmittedsimultaneously. Therefore, a UL response request (or trigger) may beprovided such that different types of UL responses to a DL MU DATA PPDUmay be transmitted at different time points. For example, to receiveblock ACK PPDUs from a plurality of STAs including UL MU transmissionsupported STAs and a UL MU transmission non-supported STA(s), the AP mayinclude an implicit block ACK request in the DL MU PPDU or an explicitblock request in a separate block ACK request PPDU, and transmit the DLMU PPDU sequentially to the UL MU transmission supported STAs and the ULMU transmission non-supported STA(s).

In the examples of FIGS. 21 and 22, it is assumed that STA1, STA2, andSTA3 are UL MU transmission supported STAs and STA4 is a UL MUtransmission non-supported STA. If the destination STAs of a DL MU DATAPPDU include both a UL MU transmission supported STA and a UL MUtransmission non-supported STA, the ACK Policy may be set to ImplicitBlock ACK Request for one or more STAs supporting one (a first type) ofdifferent UL immediate response types and to Block ACK for one or moreSTAs supporting the other type (a second type). Herein, the first typemay be the UL MU transmission-based response type and the second typemay be the UL SU transmission-based response type. Or the first type maybe the UL SU transmission-based response type and the second type may bethe UL MU transmission-based response type.

In the example of FIG. 21, the ACK Policy is set to Implicit Block ACKRequest for STA1, STA2, and STA3 supporting UL MU transmission (i.e.,supporting the UL MU transmission-based response type) in a DL MU DATAPPDU. In this case, STA1, STA2, and STA3 may transmit a UL MU Block ACKPPDU to the AP a predetermined IFS (e.g., an SIFS) after receiving theDL MU DATA PPDU. That is, the UL MU transmission supported STAs (i.e.,STAs supporting the UL MU transmission-based response type), STA1, STA2,and STA3 may simultaneously transmit block ACK frames in UL MUtransmission. Since different channel estimation sequences (e.g., HE-STFand HE-LTF sequences) are used for the plurality of STAs participatingin the UL MU transmission, the AP may receive the block ACK frames fromthe plurality of STAs without collision.

Since a UL MU transmission non-supported STA (i.e., an STA supportingonly the UL SU transmission-based response type) is not allowed totransmit a block ACK fame simultaneously with other STAs, the ACK Policymay not be set to Implicit Block ACK Request for the UL MU transmissionnon-supported STA except for the case where the ACK Policy is set toImplicit Block ACK Request for only one STA in a DL MU DATA PPDU.

In the example of FIG. 21, since the ACK Policy is set to Implicit BlockACK Request for STA1, STA2, and STA3, the ACK Policy may be set to notImplicit Block ACK Request but Block ACK for STA4 that does not supportUL MU transmission (i.e., supporting only the UL SU transmission-basedresponse type). In this case, STA4 may transmit a legacy block ACK PPDU(or a UL SU block ACK PPDU) a predetermined IFS (e.g., an SIFS) afterreceiving a block ACK request PPDU from the AP.

Meanwhile, in the example of FIG. 22, the ACK Policy is set to Block ACKfor STA1, STA2, and STA3 supporting UL MU transmission (i.e., supportingthe UL MU transmission-based response type) and to Implicit Block ACKRequest for STA4 that does not support UL MU transmission (i.e.,supporting only the UL SU transmission-based response type) in a DL MUDATA PPDU. In this case, STA4 may transmit a block ACK PPDU (e.g., alegacy block ACK PPDU or a UL SU block ACK PPDU) a predetermined IFS(e.g., an SIFS) after receiving the DL MU DATA PPDU from the AP. STA1,STA2, and STA3 may transmit a UL MU block ACK PPDU to the AP apredetermined IFS (e.g., an SIFS) after receiving a block ACK requestPPDU from the AP. That is, the UL MU transmission supported STAs (i.e.,STAs supporting the UL MU transmission-based response type), STA1, STA2,and STA3 may simultaneously transmit block ACK frames in UL MUtransmission. Since different channel estimation sequences (e.g.,different scrambling codes used in generation of HE-STFs and HE-LTFs)are used for the plurality of STAs participating in the UL MUtransmission, the AP may receive the block ACK frames from the pluralityof STAs without collision.

As described above with reference to the examples of FIGS. 21 and 22,the PPDU type of an immediate response to a DL MU PPDU (i.e., a ULresponse transmitted a predetermined IFS (e.g., an SIFS) after receptionof a DL MU PPDU) may be of the UL SU transmission type (e.g. a legacyPPDU type) or the UL MU transmission type (e.g., a UL MU PPDU type), andthe type of the immediate response to the DL MU PPDU may be determinedbased on information (e.g., information triggering UL transmission)included in the DL MU PPDU.

If the immediate response to the DL MU PPDU is of the UL MU transmissiontype, identification information about different channel estimationsequences (e.g., HE-STF and HE-LTF sequences) to be used in UL MU PPDUtransmission by a plurality of STAs may be included in the DL MU PPDU.The identification information about the channel estimation sequencesmay be defined as information indicating one element of a set includinga plurality of elements corresponding to a plurality of channelestimation sequences (or scrambling codes for generation of the channelestimation sequences). That is, the identification information about thechannel estimation sequences corresponds to information that allocatesresources (e.g., sequence resources or code resources) distinguishablefor a plurality of STAs in UL MU transmission of the STAs.

Further, the identification information about the channel estimationsequences for the UL MU transmission may be included in a frame (e.g., aDL MU PPDU) eliciting the UL MU transmission. For example, theidentification information about the channel estimation sequences forthe UL MU transmission may be included in a UL MU transmission triggerframe (i.e., a frame including information triggering a UL MUtransmission-based response) in the DL MU PPDU. Or the identificationinformation about the channel estimation sequences for the UL MUtransmission may be included in each of a plurality of PSDUs of the DLMU PPDU. More specifically, a UL MU Sequence ID subfield of the QoSControl field of the MAC header in the DL MU DATA PPDU may indicate achannel estimation sequence for the UL MU transmission. Or theidentification information (e.g., the UL MU Sequence ID subfield) aboutthe channel estimation sequences for the UL MU transmission may beincluded in a VHT Control field, a HE Control field, a Service field,etc.

If the identification information (e.g., the UL MU Sequence ID subfield)about the channel estimation sequences for the UL MU transmission has aspecific value (e.g., 0), it may indicate that a UL immediate responsePPDU transmitted a predetermined IFS (e.g., an SIFS) after reception ofthe DL MU PPDU is of the UL SU PPDU type (or the legacy PPDU type). Onthe other hand, if the identification information (e.g., the UL MUSequence ID subfield) about the channel estimation sequences for the ULMU transmission has a value other than the specific value (e.g., 0), itmay indicate that the UL immediate response PPDU transmitted apredetermined IFS (e.g., an SIFS) after reception of the DL MU PPDU isof the UL MU PPDU type. In this case, one channel estimation sequence(or one scrambling code for generation of the channel estimationsequence) may be determined from a set of a plurality of channelestimation sequences (or scrambling codes for generation of the channelestimation sequences) based on the identification information (e.g., theUL MU Sequence ID subfield) about the channel estimation sequences forthe UL MU transmission.

Referring to FIG. 21 again, the ACK Policy is set to Implicit Block ACKRequest commonly for the UL MU transmission supported STAs (i.e., STAssupporting the UL MU transmission-based response type), STA1, STA2, andSTA3, and the values of UL MU Sequence ID subfields are 1, 2, and 3,respectively for STA1, ST2, and STA3. When STA1, STA2, and STA3 transmitthe UL MU block ACK PPDU to the AP a predetermined IFS (e.g., an SIFS)after receiving the DL MU DATA PPDU, STA1, STA2, and STA3 may performthe UL MU transmission using different channel estimation sequencescorresponding to the values of the UL MU Sequence ID subfields, 1, 2,and 3. On the other hand, the value of the UL MU Sequence ID subfield isset to 0 for the UL MU transmission non-supported STA (i.e., the STAsupporting only the UL SU transmission-based response type), STA4, andSTA4 may transmit a legacy block ACK PPDU (or a UL SU block ACK PPDU) tothe AP a predetermined IFS (e.g., an SIFS) after receiving a block ACKrequest PPDU from the AP.

Referring to FIG. 22 again, the ACK Policy is set to Implicit Block ACKRequest for the UL MU transmission non-supported STA (i.e., the STAsupporting only the UL SU transmission-based response type), STA4, andthe value of the UL MU Sequence ID subfield is set to 0 for STA4. STA4may transmit a legacy block ACK PPDU (or a UL SU block ACK PPDU) to theAP a predetermined IFS (e.g., an SIFS) after receiving the DL MU DATAPPDU from the AP. On the other hand, the ACK Policy is set to ImplicitBlock ACK Request commonly for the UL MU transmission supported STAs(i.e., STAs supporting the UL MU transmission-based response type),STA1, ST2, and STA3, and the values of UL MU Sequence ID subfields are1, 2, and 3, respectively for STA1, ST2, and STA3. When STA1, STA2, andSTA3 transmit a UL MU block ACK PPDU to the AP a predetermined IFS(e.g., an SIFS) after receiving a block ACK request PPDU, STA1, STA2,and STA3 may perform the UL MU transmission using different channelestimation sequences corresponding to the values of the UL MU SequenceID subfields, 1, 2, and 3.

Like the afore-described DL MU PPDU, the block ACK request PPDU that theAP transmits to the plurality of STAs (e.g., STA1, STA2, and STA3) mayinclude information indicating the type of a UL transmission PPDU (e.g.,a UL MU transmission-based PPDU type) as an immediate response to theblock ACK request PPDU, and channel estimation sequence identificationinformation (e.g., a UL MU Sequence ID subfield) for UL MU transmissionfor each STA participating in the UL MU transmission.

Additionally, if the UL MU transmission supported STAs (i.e., the STAssupporting the UL MU transmission-based response type) simultaneouslytransmit block ACK frames in a UL MU PPDU to the AP, the transmissiontimes of the block ACK frames transmitted by the plurality of STAs maybe identical. If the transmission times of the block ACK framestransmitted simultaneously by the plurality of STAs are different, theload of processing the block ACK frames at the AP increases. To preventthe increase of the load, the transmission times of the block ACK framestransmitted by the plurality of STAs may be made identical (e.g., theblock ACK frames may be made start at the same time and end at the sametime). The same transmission time of the block ACK frames transmitted bythe plurality of STAs may mean the same transmission MCS of the blockACK frames transmitted by the plurality of STAs.

In the example of FIG. 21, to make the transmission times of the UL MUblock ACK PPDU transmitted by STA1, STA2, and STA3 supporting UL MUtransmission (i.e., the UL MU transmission-based response type)identical, MCS information may be included in the DL MU DATA PPDU. Thatis, the plurality of STAs participating in UL MU transmission may use anMCS value indicated by the MCS information for the UL MU transmission,included in the DL MU DATA PPDU, for transmission of the UL MU PPDUincluding the block ACK frames. Specifically, a UL MU MCS subfield ofthe QoS Control field of the MAC header in the DL MU DATA PPDU mayindicate an MCS value for the UL MU transmission. Or the MCS informationfor the UL MU transmission (e.g., the UL MU MCS subfield) may beincluded in a VHT Control field, a HE Control field, a Service field,etc.

In the example of FIG. 22, to make the transmission times of the UL MUblock ACK PPDUs transmitted by STA1, STA2, and STA3 supporting UL MUtransmission (i.e., the UL MU transmission-based response type)identical, MCS information for the UL MU transmission may be included inthe block ACK request PPDU.

As described before, a frame eliciting UL MU transmission may includeinformation based on which at least one of the type of a UL MU PPDU,resources for use in the UL MU transmission, and an MCS of the UL MUtransmission. That is, a frame (e.g., a DL MU DATA PPDU or a block ACKrequest PPDU for a plurality of STAs) eliciting UL MU transmission(e.g., a UL immediate response) may include at least one of informationbased on which a UL MU PPDU type (i.e., the UL MU transmission-basedPPDU type or the UL SU transmission-based PPDU type) is determined,information based on which distinguishable resources for use in the ULMU transmission (e.g., sequence resources or code resources) aredetermined, information based on which a UL MU transmission time isdetermined, and information based on which an MCS applied to the UL MUtransmission is determined.

If UL MU transmission supported STAs (i.e., STAs supporting the UL MUtransmission-based response type) simultaneously transmit block ACKframes in a UL MU PPDU, the Duration fields of the block ACK frames mayhave the same value. The value of the Duration fields included in the ULMU PPDU may be set to a value calculated by subtracting a transmissiontime of the UL MU transmission and a predetermined IFS (e.g., an SIFS)from the value of the Duration field included in the frame eliciting thetransmission of the UL MU PPDU (e.g., the DL MU DATA PPDU, or the blockACK request PPDU for a plurality of STAs). In this case, if athird-party STA receiving the UL MU PPDU determines that it is not adestination STA of the received frame even in a part of the transmissionchannel bandwidth of the UL MU PPDU (e.g., a subchannel) (e.g., if theRA value of the received frame does not match to the address of thethird-party STA), the third-party STA may perform NAV update based onthe value of the Duration field set in the above manner.

FIG. 23 depicts an exemplary method according to the present invention.

In step S2310, an AP may transmit a DL frame to an STA group includingone or more STAs. The DL frame may correspond to a DL MU DATA PPDU or ablock ACK request PPDU described in the foregoing examples. The DL framemay include information about the type of a UL frame transmitted as animmediate response (i.e., transmitted a predetermined IFS (e.g., anSIFS) after reception of the DL frame). The UL frame may correspond to ablock ACK PPDU described in the foregoing examples. The type of the ULframe may be an SU type or an MU type. If the information about the typeof a UL frame, included in the DL frame is the MU type, the DL frame mayfurther include resource allocation information, transmission timeinformation, and MCS information for a plurality of STAs, fortransmission of UL frames (i.e., a UL MU frame).

In step S2320, each STA of the STA group may determine the type of theUL frame elicited by the DL frame based on the information (e.g., theinformation about the type of a UL frame) included in the received DLframe. If the type of the UL frame is the MU type, the STA may determineUL MU transmission parameters (e.g., a resource index for the STA, fortransmission of the UL MU frame, a transmission time of the UL MU frame,and an MCS to be applied to the UL MU frame) based on the informationincluded in the DL frame (e.g., the resource allocation information, thetransmission time information, and the MCS information). Therefore, aplurality of STAs may simultaneously transmit a UL MU frame to the AP instep S2340.

If the type of the UL frame is determined to be the SU type in stepS2320, one STA may transmit a UL SU frame to the AP in step S2350.

While the afore-described exemplary methods of present invention havebeen described as a series of operations for simplicity of description,this does not limit the sequence of steps. When needed, steps may beperformed at the same time or in a different sequence. All of theexemplary steps are not always necessary to implement the methodproposed by the present invention.

The foregoing embodiments of the present invention may be implementedseparately or combinations of two or more of the embodiments may beimplemented simultaneously, for the afore-described exemplary methods ofpresent invention.

The present invention includes an apparatus for processing or performingthe method of the present invention (e.g., the wireless device and itscomponents described with reference to FIGS. 1, 2, and 3).

The present invention includes software (an operating system (OS), anapplication, firmware, a program, etc.) for executing the method of thepresent invention in a device or a computer, and a medium storing thesoftware that can be executed in a device or a computer.

While various embodiments of the present invention have been describedin the context of an IEEE 802.11 system, they are applicable to variousmobile communication systems.

What is claimed is:
 1. A method for transmitting an uplink frame by a station (STA) to an access point (AP) in a wireless local area network, the method comprising: receiving, from the AP, a downlink frame including information related to a type of the uplink frame, the type of the uplink frame including a single-user (SU) type and a multiple-user (MU) type; and transmitting, to the AP, the uplink frame having a type determined based on the information related to the type of the uplink frame, wherein, when the type of the uplink frame corresponds to the MU type, the uplink frame is simultaneously transmitted by a plurality of STAs including the STA and at least one other STA.
 2. The method according to claim 1, wherein the downlink frame further includes resource allocation information for the plurality of STAs.
 3. The method according to claim 2, wherein the resource allocation information includes information indicating distinguished resources for each of the plurality of STAs.
 4. The method according to claim 1, wherein the downlink frame further includes Modulation and Coding Scheme (MCS) information for the uplink frame.
 5. The method according to claim 4, wherein a same MCS based on the MCS information is applied to the uplink frame by the plurality of STAs.
 6. The method according to claim 1, wherein, when the type of the uplink frame corresponds to the SU type, the uplink frame is transmitted only by the STA.
 7. The method according to claim 1, wherein the uplink frame is transmitted as an immediate response to the downlink frame.
 8. The method according to claim 7, wherein the immediate response is transmitted a Short Inter-Frame Space (SIFS) time after the downlink frame.
 9. The method according to claim 1, wherein the downlink frame includes downlink data for the plurality of STAs.
 10. The method according to claim 1, wherein the downlink frame includes block acknowledgement (ACK) requests for the plurality of STAs.
 11. A method for receiving an uplink frame by an access point (AP) from at least one station (STA) in a wireless local area network, the method comprising: transmitting, to the at least one STA, a downlink frame including information related to a type of the uplink frame, the type of the uplink frame including a single-user (SU) type and a multiple-user (MU) type; and receiving, from the at least one STA, the uplink frame having a type determined based on the information related to the type of the uplink frame, wherein, when the type of the uplink frame corresponds to the MU type, the uplink frame is simultaneously transmitted by a plurality of STAs including the at least one STA.
 12. The method according to claim 11, wherein the downlink frame further includes resource allocation information for the plurality of STAs.
 13. The method according to claim 12, wherein the resource allocation information includes information indicating distinguished resources for each of the plurality of STAs.
 14. The method according to claim 11, wherein the downlink frame further includes Modulation and Coding Scheme (MCS) information for the uplink frame.
 15. The method according to claim 14, wherein a same MCS based on the MCS information is applied to the uplink frame by the plurality of STAs.
 16. The method according to claim 11, wherein, when the type of the uplink frame corresponds to the SU type, the uplink frame is transmitted only by a STA.
 17. The method according to claim 11, wherein the uplink frame is received as an immediate response to the downlink frame.
 18. The method according to claim 17, wherein the immediate response is received a Short Inter-Frame Space (SIFS) time after the downlink frame.
 19. The method according to claim 11, wherein the downlink frame includes downlink data for the plurality of STAs.
 20. The method according to claim 11, wherein the downlink frame includes block acknowledgement (ACK) requests for the plurality of STAs. 